implement union of two PyCOMPS by calling into the C.

instead of reimplementing in the bindings.
pycomps.c: Nobobdy uses the module_state struct.

it actually yields a compiler warning as the type is static.
fix: sigsegv in comps_doc_union() for a fresh COMPS_Doc without content.

Fixes https://github.com/midnightercz/libcomps/issues/3.