impl Trait not allowed outside of function and method return types

… but it is now!

This library gives you one macro, and one macro only: #[desugar_impl].

Annotate any struct, enum, or union with #[desugar_impl] to allow the use of field_name: impl SomeTrait in their declaration. E.g.

```

[desugar_impl]

struct Test { a: impl Clone + PartialOrd, b: impl Clone + PartialOrd, c: impl Hash, } ```

desugars to

struct Test<Ty1, Ty2, Ty3> where Ty1: Clone + PartialOrd, Ty2: Clone + PartialOrd, Ty3: Hash, { a: Ty1, b: Ty2, c: Ty3, }

You can still place any #[derive(…)] macros just below #[desugar_impl] any they see work with the desugared code.