Sync Python bindings with latest C API changes

Add gcc_jit_object, block, field, and struct wrappers.
Remove gcc_jit_label and loop.

Add doc strings to all functions, currently only show the function
signature.

Clean up some redundant value creation code by adding: RValue_from_c
and similar variations.

Change Result.get_code() to return an unsigned long back to Python.
This way we can wrap different ctypes call signatures around the
resulting pointer.

Attempt to keep order of definitions in gccjit.pxd the same as
they are defined in gccjit.h.
Add all gcc_jit_comparison enum values.

Nest all enum values under named classes.