libgolang: Provide Nil as alias for std::nullptr_t

This continues 60f6db6f (libgolang: Provide nil as alias for nullptr and
NULL): I've tried to compile pygolang with Clang on my Debian 10
workstation and got:

    $ CC=clang CXX=clang++ python setup.py build_dso -i

    In file included from ./golang/fmt.h:32:
    ./golang/libgolang.h:381:11: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'?
    constexpr nullptr_t nil = nullptr;
              ^~~~~~~~~
              std::nullptr_t
    /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/x86_64-linux-gnu/c++/8/bits/c++config.h:242:29: note: 'std::nullptr_t' declared here
      typedef decltype(nullptr)     nullptr_t;
                                    ^
    :
    In file included from ./golang/context.h
    In file included from golang/runtime/libgolang.cpp:30:
    ./golang/libgolang.h:381:11: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'?
    constexpr nullptr_t nil = nullptr;
              ^~~~~~~~~
              std::nullptr_t
    /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/x86_64-linux-gnu/c++/8/bits/c++config.h:242:29: note: 'std::nullptr_t' declared here
      typedef decltype(nullptr)     nullptr_t;
                                    ^
    :39:
    ./golang/libgolang.h:381:11: error: unknown type In file included from golang/fmt.cpp:25:
    In file included from ./golang/fmt.h:32:
    ./golang/libgolang.h:421:17: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'?
        inline chan(nullptr_t) { _ch = nil; }
                    ^~~~~~~~~
                    std::nullptr_t

    ...

It seems with GCC and Clang under macOS nullptr_t is automatically provided in
builtin namespace, while with older Clang on Linux (clang version 7.0.1-8) only
in std:: namespace - rightfully as nullptr_t is described to be present there:

https://en.cppreference.com/w/cpp/types/nullptr_t

This way we either have to correct all occurrences of nullptr_t to
std::nullptr_t, or do something similar with providing nil under golang:: .

To reduce noise I prefer the later and let it be named as Nil.

golang/_context.pxd

@@ -34,7 +34,7 @@ See also https://blog.golang.org/context for overview.
 (*) not provided in Go version.
 """
 
-from golang cimport chan, structZ, error, refptr, interface
+from golang cimport chan, structZ, error, refptr, interface, Nil
 from golang cimport cxx
 from libcpp.utility cimport pair
 
@@ -44,7 +44,7 @@ from libcpp.utility cimport pair
 cdef extern from "golang/libgolang.h" namespace "golang" nogil:
     cppclass cancelFunc "golang::func<void()>":
         void operator() ()
-        void operator= (nullptr_t)
+        void operator= (Nil)
 
 cdef extern from "golang/context.h" namespace "golang::context" nogil:
     cppclass _Context:

golang/_golang.pxd

@@ -44,7 +44,7 @@ In addition to Cython/nogil API, golang.pyx provides runtime for golang.py:
 """
 
 
-from libcpp cimport nullptr_t, nullptr as nil # golang::nil = nullptr
+from libcpp cimport nullptr_t as Nil, nullptr as nil # golang::nil = nullptr
 from libcpp.utility cimport pair
 from libc.stdint cimport uint64_t
 cdef extern from *:
@@ -88,9 +88,9 @@ cdef extern from "golang/libgolang.h" namespace "golang" nogil:
         unsigned cap()                      const
 
         # compare wrt nil; =nil
-        cbool operator==(nullptr_t)         const
-        cbool operator!=(nullptr_t)         const
-        void operator=(nullptr_t)
+        cbool operator==(Nil)               const
+        cbool operator!=(Nil)               const
+        void operator=(Nil)
 
         # for tests
         _chan *_rawchan()   const
@@ -127,9 +127,9 @@ cdef extern from "golang/libgolang.h" namespace "golang" nogil:
     # memory management of C++ nogil classes
     cppclass refptr[T]:
         # compare wrt nil; =nil
-        cbool operator== (nullptr_t)    const
-        cbool operator!= (nullptr_t)    const
-        void  operator=  (nullptr_t)    const
+        cbool operator== (Nil)          const
+        cbool operator!= (Nil)          const
+        void  operator=  (Nil)          const
 
         # compare wrt refptr; =refptr
         # XXX workaround for https://github.com/cython/cython/issues/1357:

golang/libgolang.h

@@ -366,6 +366,7 @@ LIBGOLANG_API extern void (*_tblockforever)(void);
 #ifdef __cplusplus
 
 #include <atomic>
+#include <cstddef>
 #include <exception>
 #include <functional>
 #include <initializer_list>
@@ -377,8 +378,9 @@ LIBGOLANG_API extern void (*_tblockforever)(void);
 
 namespace golang {
 
-// nil is alias for nullptr and NULL.
-constexpr nullptr_t nil = nullptr;
+// nil is alias for nullptr and NULL; Nil - for std::nullptr_t;
+using Nil = std::nullptr_t;
+constexpr Nil nil = nullptr;
 
 // string is alias for std::string.
 using string = std::string;
@@ -418,8 +420,8 @@ public:
     inline ~chan() { _chanxdecref(_ch); _ch = nil; }
 
     // = nil
-    inline chan(nullptr_t) { _ch = nil; }
-    inline chan& operator=(nullptr_t) { _chanxdecref(_ch); _ch = nil; return *this; }
+    inline chan(Nil) { _ch = nil; }
+    inline chan& operator=(Nil) { _chanxdecref(_ch); _ch = nil; return *this; }
     // copy
     inline chan(const chan& from) { _ch = from._ch; _chanxincref(_ch); }
     inline chan& operator=(const chan& from) {
@@ -466,8 +468,8 @@ public:
     inline unsigned cap()       const  { return _chancap(_ch); }
 
     // compare wrt nil
-    inline bool operator==(nullptr_t) const  { return (_ch == nil); }
-    inline bool operator!=(nullptr_t) const  { return (_ch != nil); }
+    inline bool operator==(Nil) const  { return (_ch == nil); }
+    inline bool operator!=(Nil) const  { return (_ch != nil); }
 
     // compare wrt chan
     inline bool operator==(const chan<T>& ch2) const { return (_ch == ch2._ch); }
@@ -587,8 +589,8 @@ public:
     }
 
     // = nil
-    inline refptr(nullptr_t)        { _obj = nil;  }
-    inline refptr& operator=(nullptr_t) {
+    inline refptr(Nil)              { _obj = nil;  }
+    inline refptr& operator=(Nil) {
         if (_obj != nil)
             _obj->decref();
         _obj = nil;
@@ -632,8 +634,8 @@ public:
     friend refptr<T> newref<T>  (T *_obj);
 
     // compare wrt nil
-    inline bool operator==(nullptr_t) const { return (_obj == nil); }
-    inline bool operator!=(nullptr_t) const { return (_obj != nil); }
+    inline bool operator==(Nil) const { return (_obj == nil); }
+    inline bool operator!=(Nil) const { return (_obj != nil); }
 
     // compare wrt refptr
     inline bool operator==(const refptr& p2) const { return (_obj == p2._obj);  }
@@ -691,8 +693,8 @@ public:
     }
 
     // = nil
-    inline global(nullptr_t)        { _obj = nil;   }
-    inline global& operator=(nullptr_t) {
+    inline global(Nil)              { _obj = nil;   }
+    inline global& operator=(Nil) {
         if (_obj != nil)
             _obj->decref();
         _obj = nil;
@@ -703,8 +705,8 @@ public:
     // move - no need due to refptr<T> cast
 
     // compare wrt nil
-    inline bool operator==(nullptr_t) const { return (_obj == nil); }
-    inline bool operator!=(nullptr_t) const { return (_obj != nil); }
+    inline bool operator==(Nil) const { return (_obj == nil); }
+    inline bool operator!=(Nil) const { return (_obj != nil); }
 
     // compare wrt refptr
     inline bool operator==(const refptr<T>& p2) const { return (_obj == p2._obj);  }