libctf: create: structure and union member addition

There is one API addition here:

int ctf_add_member_bitfield (ctf_dict_t *, ctf_id_t souid,
                             const char *, ctf_id_t type,
                             unsigned long bit_offset,
                             int bit_width);

SoU addition handles the representational changes for bitfields and for
CTF_K_BIG structs (i.e. all structs you can add members to), errors out if
you add bitfields to structs that aren't created with the
CTF_ADD_STRUCT_BITFIELDS flag, and arranges to add padding as needed if
there is too much of a gap for the offsets to encode in one hop (that
part is still untested).

libctf/libctf.ver

@@ -132,6 +132,7 @@ LIBCTF_2.0 {
 	ctf_add_member_offset;
 	ctf_add_member_encoded;
 	ctf_add_variable;
+	ctf_add_member_bitfield;
 
 	ctf_set_array;