gitypeinfo: Add pointer-stuffing functions for GITypeTag storage type

This adds gi_type_tag_argument_from_hash_pointer() and
gi_type_tag_hash_pointer_from_argument(). They do the same thing as
the corresponding g_type_info_... functions, which are used to pack and
unpack the correct field of a GIArgument into/from a data pointer in
GHashTable or GList, regardless of machine architecture or endianness.

These functions take a GITypeTag obtained from
g_type_info_get_storage_type(), instead of a GITypeInfo pointer. (The
storage type is the only piece of data that is actually used from the
GITypeInfo structure.)

It's intended for bindings using an argument cache, such as GJS and
PyGObject, so that they don't have to store a whole 64-bit GITypeInfo
pointer in their cache in many common cases, and can just store the 5-bit
type tag instead.

The original g_type_info_... functions are reimplemented in
terms of the new g_type_tag... functions.