* Initializing PyModuleDef object with PyModuleDef_HEAD_INIT.
Python 3.8 documentation: m_base - Always initialize this member to PyModuleDef_HEAD_INIT.
Long-standing (since first github commit in 2015), inconsequential bug.
Also removing inconsequential Py_INCREF(def): PyModule_Create() resets the reference count to 1.
* git rebase master
* moving static PyModuleDef declaration to global scope, as requested by @wjakob
* renaming the two new macros, to start with PYBIND11_DETAIL_MODULE
