Checkpoint pcwalton's revisions to deal with 8861 fallout.
pcwalton's addition of `ReFunction` to enum `ty::Region`.
Checkpoint pcwalton's dtor checker with `SafeDestructor` in enum `infer::SubregionOrigin`.

Must rename breadcrumbs, or at least improve its documentation.
checkpoint pcwalton factoring out of `fn mem_categorization::scope`.
checkpoint pcwalton removal of `#[unsafe_destructor]` check.
checkpoint pcwalton test of new destructor static semantics.