#[cgp_dispatch] macro improvements

[soareschen]

Summary

This is a follow up on #139 and adds a couple of minor improvements on the #[cgp_dispatch] macro to support more corner cases.

• The dispatcher trait can now support multiple trait methods
• Async functions are now properly supported
• Generic parameters can be specified on the dispatcher trait, provided that the parameter is used in the body.
• Non-lifetime generic parameters inside trait methods are not supported, due to the lack of support in Rust for non-lifetime higher-ranked trait bounds, a.k.a. quantified constraints.