Add validation for the signatures of functions annotated with #[event(fetch)], #[event(start)] etc.

[jakubadamw]

This addresses an old TODO by having the signatures for user-defined event handlers be validated at the proc macro expansion stage to avoid having the errors crop up during type check of the expanded code in a form that wasn't exactly always very clear, like so:

error[E0061]: this function takes 1 argument but 0 arguments were supplied
 --> tests/ui/start-wrong-param-count.rs:4:4
  |
3 | #[event(start)]
  | --------------- argument #1 of type `u32` is missing
4 | fn start(a: u32) {}
  |    ^^^^^
  |

WIth this change the macro reports:

error: custom attribute panicked
 --> tests/ui/start-wrong-param-count.rs:3:1
  |
3 | #[event(start)]
  | ^^^^^^^^^^^^^^^
  |
  = help: message: the `start` handler expects 0 argument(s) but found 1

I also added a few tests using the trybuild crate.