This refactors the GJS_DEFINE_PROTO family of macros to store the created
prototype objects in slots on the global object. We split up the
gjs_WHATEVER_create_proto() function into two functions: one that must be
called at least once for each type to define the class, store the prototype
in a global slot, and define the constructor as a property on a passed-in
module or global object, gjs_WHATEVER_define_proto(); and one that
retrieves the prototype from its global slot, gjs_WHATEVER_get_proto().

We also add two macros, GJS_DEFINE_PROTO_WITH_PARENT and
GJS_DEFINE_PROTO_ABSTRACT_WITH_PARENT, which also move the definition of
the parent to compile time rather than runtime, and keep it in the same
place as the rest of each class's macro definition. Similarly, previously
we had to pass the parent prototype object to gjs_WHATEVER_create_proto().

This mostly affects the cairo module.

https://bugzilla.gnome.org/show_bug.cgi?id=614413