Rollup merge of #51104 - zackmdavis:dynamo, r=nikomatsakis

add `dyn ` to display of dynamic (trait) types

~~I'm not sure we want the `dyn` in the E0277 "trait bound [...] is not satisfied" messages ("bound" sounds like a different thing in contrast to the names of specific trait-object types like `Box<dyn Trait>`), but I'm finding the code I would need to change that hard to follow—the [display object seems to](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/traits/error_reporting.rs#L600) be a [`Predicate::Trait`](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/ty/mod.rs#L962) variant, whose [`Display` implementation](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/util/ppaux.rs#L1309) calls `.print` on its `PolyTraitPredicate` member, [which is a type alias](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/ty/mod.rs#L1112) for `ty::Binder<TraitPredicate<'tcx>>`, whose [`Display` implementation](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/util/ppaux.rs#L975-L985) ... _&c._— so maybe it's time to pull-request this and see what reviewers think.~~

 Resolves #49277 (?).

r? @nikomatsakis