libgolang: Provide nil as alias for nullptr and NULL

Nil is more native to Go.

golang/_golang.pxd

 # cython: language_level=2
-# Copyright (C) 2019  Nexedi SA and Contributors.
-#                     Kirill Smelkov <kirr@nexedi.com>
+# Copyright (C) 2019-2020  Nexedi SA and Contributors.
+#                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
 # it under the terms of the GNU General Public License version 3, or (at your
@@ -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
+from libcpp cimport nullptr_t, nullptr as nil # golang::nil = nullptr
 from libcpp.utility cimport pair
 from libc.stdint cimport uint64_t
 cdef extern from *:

golang/libgolang.h

 #ifndef _NXD_LIBGOLANG_H
 #define _NXD_LIBGOLANG_H
 
-// Copyright (C) 2018-2019  Nexedi SA and Contributors.
+// Copyright (C) 2018-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -377,6 +377,9 @@ LIBGOLANG_API extern void (*_tblockforever)(void);
 
 namespace golang {
 
+// nil is alias for nullptr and NULL.
+constexpr nullptr_t nil = nullptr;
+
 // string is alias for std::string.
 using string = std::string;