js: Implement barriers correctly in structs

Fundamental was the only class that didn't have the IMPLEMENTS_BARRIERS
flag set. In SpiderMonkey 38 it's required on all classes that have a
trace hook.

In addition, in order to implement barriers correctly we need to make sure
that JS::Heap's constructor and destructor are called. In most cases we do
this by making the containing struct a C++ struct, and calling its
constructor and destructor. In the case of GjsContext (a GObject struct)
and GjsCoveragePrivate (a GObject private struct) we can't do that, so
instead we use placement new on the JS::Heap member.

https://bugzilla.gnome.org/show_bug.cgi?id=776966