std,extra: Make some types public and other private.

These are either returned from public functions, and really should
appear in the documentation, but don't since they're private, or are
implementation details that are currently public.
auto merge of #11808 : huonw/rust/std-visible-types, r=brson

These are either returned from public functions, and really should
appear in the documentation, but don't since they're private, or are
implementation details that are currently public.