This somewhat heavyweight solution will avoid size_t/long long/long/int
mismatches on various platforms once and for all. The previous template
overloads could e.g. not handle size_t on Darwin.

One gotcha: the 'format_descriptor<T>::value()' syntax changed to just
'format_descriptor<T>::value'

This is needed in case an older pybind11 version was installed with pip

mkdoc.py: fix typo in `__DOC5` macro

This prevented `DOC(...)` calls with 5 arguments to resolve correctly.