In order to set and get enum and flag fields in structures, we need
to know the integral type that the enumeration is stored as. We are already
computing that at compile time in order to compute struct offsets, so the
easiest thing to do is to save that in the typelib.

* docs/typelib-format.txt girepository/girnode.[ch] girepository/giroffsets.c
girepository/gtypelib.h: Add 'storage_type' to the typelib format for EnumBlob
and compute and save it at compile time.

* girepository/girepository.h girepository/ginfo.c:
Add g_enum_info_get_storage_type().

* girepository/gfield.c: Implement reading and writing enum and flags fields
based on the storage type.