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
Flag enums are already treated as a special kind of enum within
`GIInfoType`, so it would be tidier to give it its own `GType` too, with
a subtype relation to `GI_TYPE_ENUM_INFO`.
This will simplify implementing `GI_IS_ENUM_INFO` in a following commit
too.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #3216
