Refactor TypeContents to be more scrutable. In particular, create coarser bits
than the current ones, which were very fine-grained.  Also, cleanly distinguish
when properties must be found in *owned* types vs *reachable* types.

Fixes #10157
Fixes #10278
Make TypeContents consider the type `T` to be reachable via `*T` pointers

Fixes #9509
auto merge of #10289 : nikomatsakis/rust/issue-10157-TypeContents-refactor, r=pcwalton

See #10157. This compiles now.

Fixes #10278.