These follow GObject conventions, using `G_TYPE_CHECK_INSTANCE_CAST` to
cast to the given type, and potentially performing some runtime checks
of the type instance’s `GType` too.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Fixes: #3216
Boxed types are already represented within `GIInfoType`, so they should
have a `GType` representation as well.
In an upcoming commit, this will allow us to represent the subtype
relation between `GIBoxedInfo` and `GIRegisteredTypeInfo` too.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #3216
