Rollup merge of #56045 - qnighy:additional-sizedness, r=cramertj

Check arg/ret sizedness at ExprKind::Path

This PR solves three problems:

- #50940: ICE on casting unsized tuple struct constructors
- Unsized tuple struct constructors were callable in presence of `unsized_locals`.
- https://github.com/rust-lang/rust/issues/48055#issuecomment-437178966: we cannot relax `Sized` bounds on stable functions because of fn ptr casting

These are caused by lack of `Sized`ness checks for arguments/retvals at **reference sites of `FnDef` items** (not call sites of the functions). Therefore we can basically add more `Sized` obligations on typeck. However, adding `Sized` obligations arbitrarily breaks type inference; to prevent that I added a new method `require_type_is_sized_deferred` which doesn't interfere usual type inference.

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