Dramatically reduce the size of libphosh public API

The GIR and libphosh.h are reworked to only include a very small set of headers for the current minimum viable public API (based on existing usage in libphosh-rs examples and phrog).

The GIR has been shrunk from ~70k lines to ~5k, and the number of files installed to /usr/include/libphosh-N.NN has reduced from ~200 down to ~20.

I've tested the results of this patch series against libphosh-rs and phrog and everything seems to work okay. I'm not sure if you've got any other libphosh(?!-rs) consumers lying around that might be relying on stuff that has now been removed from the public API, though.