libgolang/{thread,gevent}: Switch runtimes to C++

We will soon need to use common functionality(*) from both runtimes and
other packages. The "other packages" are all C++ and it is handy to keep
common functionality in C++ as well. While we could also maintain
`extern "C"` interface, it duplicates the work. Let's switch everything
to C++ to ease further maintenance.

(*) e.g. package internal/syscall from both runtimes and from package os.

/reviewed-on !17

golang/runtime/.gitignore

-/_runtime_gevent.c
-/_runtime_thread.c
+/_runtime_gevent.cpp
+/_runtime_thread.cpp

golang/runtime/_libgolang.pxd

 # cython: language_level=2
-# Copyright (C) 2019  Nexedi SA and Contributors.
+# Copyright (C) 2019-2022  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -21,7 +21,7 @@
 
 from libc.stdint cimport uint64_t
 
-cdef extern from "golang/libgolang.h" nogil:
+cdef extern from "golang/libgolang.h" namespace "golang" nogil:
     struct _libgolang_sema
     enum _libgolang_runtime_flags:
         STACK_DEAD_WHILE_PARKED

setup.py

 # pygolang | pythonic package setup
-# Copyright (C) 2018-2020  Nexedi SA and Contributors.
+# Copyright (C) 2018-2022  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -233,12 +233,10 @@ setup(
                         ['golang/_golang.pyx']),
 
                     Ext('golang.runtime._runtime_thread',
-                        ['golang/runtime/_runtime_thread.pyx'],
-                        language = "c"),
+                        ['golang/runtime/_runtime_thread.pyx']),
 
                     Ext('golang.runtime._runtime_gevent',
-                        ['golang/runtime/_runtime_gevent.pyx'],
-                        language = 'c'),
+                        ['golang/runtime/_runtime_gevent.pyx']),
 
                     Ext('golang.pyx.runtime',
                         ['golang/pyx/runtime.pyx'],