When overriding visit_edges() in a JS::Object subclass, we must make
sure to call the base class visit_edges(), or the object's Shape (and
any properties) will not get marked.
This is Abstract Operation is required for the majority of
InstantConstructor's and InstantPrototype's methods.
The implementation is not entirely complete, (specifically 2 of the
underlying required abstract operations, ParseTemporalTimeZoneString
and ParseISODateTime are missing the required lexing, and as such are
TODO()-ed) but the majority of it is done.
