This part replaces the JSContext *cx argument in most GC-related API
with JSRuntime *rt. When possible, the patch removes the code to obtain
a temporary cx just to run the GC.

The patch also removes JS_DestroyContextMaybeGC. That function is not
used in FF code base and its implementation is broken. It requires that
the context has an entered compartment when it is destroyed, which in
turns implies a missing leave compartment call.