Replace C-style _PyJITContext with C++-style jit::Context

Summary:
No intended behavior changes, just refactoring this POD struct with a lot of
associated functions into a proper C++ class.

This diff removes the `get{Code,Stack,SpillStack}Size()` methods and moves their
logic into the single callsites where they are used (pyjit.cpp).  They would all
be wrappers around the `lookupFunc()` method, so that is made public.

Reviewed By: swtaarrs

Differential Revision: D52684596

fbshipit-source-id: eadd0d489a2099ebbbcc84d01c1f4c78d1cc81a2