libctf: types: add ctf_struct_bitfield (NEEDS REVIEW)

libctf: types: add ctf_struct_bitfield (NEEDS REVIEW)

This new public API function allows you to find out if a struct has the
bitfield flag set or not.  (There are no other properties specific to a
struct, so we needed a new function for it.  I am open to a
ctf_struct_info() function handing back a struct if people prefer.)

New API:

int ctf_struct_bitfield (ctf_dict_t *, ctf_id_t);