libctf, create, types: type and decl tags

libctf, create, types: type and decl tags

These are a little more fiddly than previous kinds, because their
namespacing rules are odd: they have names (so presumably we want an API to
look them up by name), but the names are not unique (they don't need to be,
because they are not entities you can refer to from C), so many distinct
tags in the same TU can have the same name.  Type tags only refer to a type
ID: decl tags refer to a specific function parameter or structure member via
a zero-indexed "component index".

The name tables for these things are a hash of name to a set of type IDs;
rather different from all the other named entities in libctf.  As a
consequence, they can presently be looked up only using their own dedicated
functions, not using ctf_lookup_by_name et al.  (It's not clear if this
restriction could ever be lifted: ctf_lookup_by_name and friends return a
type ID, not a set of them.)

They are similar enough to each other that we can at least have one function
to look up both type and decl tags if you don't care about their
component_idx and only want a type ID: ctf_tag.  (And one to iterate over
them, ctf_tag_next).

(A caveat: because tags aren't widely used or generated yet, much of this is
more or less untested and/or supposition and will need testing later.)

New API, more or less the minimum needed because it's not entirely clear how
these things will be used:

+ctf_id_t ctf_tag (ctf_dict_t *, ctf_id_t tag);
+ctf_id_t ctf_decl_tag (ctf_dict_t *, ctf_id_t decl_tag,
+		       int64_t *component_idx);
+ctf_id_t ctf_tag_next (ctf_dict_t *, const char *tag, ctf_next_t **);
+ctf_id_t ctf_add_type_tag (ctf_dict_t *, uint32_t, ctf_id_t, const char *);
+ctf_id_t ctf_add_decl_type_tag (ctf_dict_t *, uint32_t, ctf_id_t, const char *);
+ctf_id_t ctf_add_decl_tag (ctf_dict_t *, uint32_t, ctf_id_t, const char *,
+			   int component_idx);