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.