Support generics on the trait

[tikue]

Technically non-'static requests are supported today, but it's difficult to reference them in a trait item if the trait isn't parameterized by the lifetime. More generally, it may be worth supporting type parameters on the declared traits.

[tikue]

I looked into this briefly, and it appears complex.

• need to handle where clauses and trait bounds
• need to figure out if type params apply to requests or responses.
  • if it applies to responses, it needs to apply to the response future as well.