feat: support generic fn params without static bound

[audunhalland]

fixes #52

This is the implementation for #52, which supports fn generic bounds like fn method<T>(&self, arg: T), where T does not have a 'static bound.

It is technically a breaking change, because more trait definitions now require explicit 'static bounds, e.g. the code example from #37. Even if the bound trait that has an implicit 'static super-bound, unimock now requires an extra explicit bound regardless.

I'm unsure if this change is desired, so the PR will be left open for a while.