Embed Provider_context.t in Typing_classes_heap.t

Embed Provider_context.t in Typing_classes_heap.t

Summary:
When calling the `Cls.{construct,method,smethod,prop,sprop}` accessors on `Typing_classes_heap.t` we will need a `Provider_context.t` if we want to access the shallow decl in the event that the member signature was evicted from the member heaps/worker cache.

There are two ways to do this:

  1. The approach this diff takes: embed a `Provider_context.t` in each `Typing_class_heap.t`
  2. Have all `Cls.{construct,method,...}` take an additional `ctx` argument and fixup all callsites.

Unfortunately, (2) is not cleanly feasible. Zoncolan abstracts over Hack's decl provider and none of the callsites have access to a `Provider_context.t`. Refactoring this would be hellish.

For now the embedded `ctx` is not yet used. See next diffs

Reviewed By: CatherineGasnier

Differential Revision: D33897157

fbshipit-source-id: d89e515e3f08a26203300e02cce0c7a6ce15c8b0