RFC: Change DST syntax to `T: Sized?`
Add `T: ?Sized` as an alternative
Make changes as requested by the Rust team

This adjusts the proposal to propose `T: ?Sized` instead of `T: Sized?`, and
changes the syntax for traits to `trait Trait for ?Sized` instead of `trait
Trait: Sized?`.
Merge branch 'dst-syntax' of https://github.com/P1start/rfcs into P1start-dst-syntax
Merge branch 'P1start-dst-syntax'