Reorganize class.h

Summary: Ain't she a beaut'?  Organize and document all of class.h, and spawn off
a class-inl.h.

I'll organize class.cpp also, shortly before I start working on
rejiggering the loader logic.  This may not happen immediately, but this
cleanup is still good now.

Reviewed By: @jdelong

Differential Revision: D1399915