Deriving TypeGenerator requires that it is in scope

[Ekleog]

I usually do not use the derive-macros that I use, so that I always know which crate they're coming from.

However, if I #[derive(bolero_generator::TypeGenerator)], rustc complains that it's referring to a not-in-scope TypeGenerator.

Ideally, TypeGenerator would refer to a full-path variant of itself, to make possible such derives.

That said it's certainly not a big deal, as the fix is simple and quite obvious, I'm just submitting this here as a probably-good-first-issue area of improvement :)