Support for generic fields

[hobofan]

As of right now the crate doesn't support generic fields / trait bounds on the implementer.

E.g.:

#[derive(Delegate)]
#[delegate(Shout, target = "foo")]
pub struct WrappedAnimals<A: Shout> {
    foo: Cat,
    bar: A,
}

will result in the error:

error[E0107]: wrong number of type arguments: expected 1, found 0
  --> src/main.rs:28:12
   |
28 | pub struct WrappedAnimals<A: Shout> {
   |            ^^^^^^^^^^^^^^ expected 1 type argument