[RFC] refactor includes + improve testing infrastructure

This will allows testing of more files and allow PR #904 to proceed.

Implements a tiny part of issue #918 in the process as well.
Looks like Vim uses `__int64` on win64: https://github.com/b4winckler/vim/blob/master/src/vim.h#L397-L420

So this comment makes no sense for Neovim currently, but we'll need to address it eventually, of course.