libctf: rename the type_mapping_key to type_key

libctf: rename the type_mapping_key to type_key

The name was just annoyingly long and I kept misspelling it.
It's also a bad name: it's not a mapping the type might be *used* in a
type mapping, but it is itself a representation of a type (a ctf_file_t
/ ctf_id_t pair), not of a mapping at all.

libctf/
	* ctf-impl.h (ctf_link_type_mapping_key): Rename to...
	(ctf_link_type_key): ... this, adjusting member prefixes to
	match.
	(ctf_hash_type_mapping_key): Rename to...
	(ctf_hash_type_key): ... this.
	(ctf_hash_eq_type_mapping_key): Rename to...
	(ctf_hash_eq_type_key): ... this.
	* ctf-hash.c (ctf_hash_type_mapping_key): Rename to...
	(ctf_hash_type_key): ... this, and adjust for member name
	changes.
	(ctf_hash_eq_type_mapping_key): Rename to...
	(ctf_hash_eq_type_key): ... this, and adjust for member name
	changes.
	* ctf-link.c (ctf_add_type_mapping): Adjust.  Note the lack of
	need for out-of-memory checking in this code.
	(ctf_type_mapping): Adjust.