`impl trait` in bindings (feature: impl-trait-existential-types)
This PR enables `impl Trait` syntax (opaque types) to be used in bindings, e.g.
* `let foo: impl Clone = 1;`
* `static foo: impl Clone = 2;`
* `const foo: impl Clone = 3;`
This is part of [RFC 2071](https://github.com/rust-lang/rfcs/blob/master/text/2071-impl-trait-existential-types.md) ([tracking issue](https://github.com/rust-lang/rust/issues/34511)), but exists behind the separate feature gate `impl_trait_in_bindings`.
CC @cramertj @oli-obk @eddyb @Centril @varkor
4141a4079e