Convert C++ part of the project to use nil instead of NULL/nullptr.

We do not convert pyx part yet, because Cython currently does not
understand that nullptr_t has properties of NULL and with e.g. the
following change

    --- a/golang/_context.pyx
    +++ b/golang/_context.pyx
    @@ -116,7 +116,7 @@ cdef cppclass _PyValue (_interface, gobject) nogil:
         __dealloc__():
             with gil:
                 obj = <object>this.pyobj
    -            this.pyobj = NULL
    +            this.pyobj = nil
                 Py_DECREF(obj)

errors as

    Error compiling Cython file:
    ------------------------------------------------------------
    ...
            if __decref():
                del self
        __dealloc__():
            with gil:
                obj = <object>this.pyobj
                this.pyobj = nil
                            ^
    ------------------------------------------------------------

    golang/_context.pyx:119:25: Cannot assign type 'nullptr_t' to 'PyObject *'

https://github.com/cython/cython/issues/3314