`box` and `in` expressions (tracking issue for RFC 809)

[nikomatsakis]

Tracking issue for rust-lang/rfcs#809. This should be filled out with TODO items. Note that in this case there are some unsettled design questions to be resolved as well.

To do:

• land placement-in desugaring implementation
• land overloaded-box desugaring implementation
• port libstd APIs to support in
• port libstd APIs to support box
• resolve/finalize syntax for placement-in
• investigate improvement of inference, coercion, and cast interactions (subissues could type inference fill in the _ in expr as Box<_> when expecting Box<Trait> ? #22221, Cannot do x as Box<[T]> for x: Box<[T; k]> #22403, Could type inference insert coercions to accommodate box desugaring? #22405)
• improve error messages when code is rejected (box/in desugaring leads to inscrutable error messages #22231)

Unresolved questions:

• best precedence for box (was Precedence of box is possibly wrong #21192)

[pnkfelix]

Status update: https://internals.rust-lang.org/t/lang-team-minutes-feature-status-report-placement-in-and-box/4646