Make Fn* types work with some generic methods

[LukasKalbertodt]

Currently, this:

#[auto_impl(Fn)]
trait Greeter {
    fn greet<T: Display>(&self, name: T);
}

Generates this:

impl<U: ::std::ops::Fn(T)> Greeter for U {
    fn greet<T: Display>(&self, name: T) {
        ({ self })(name)
    }
}

This doesn't compile as T is only declared on the method but already used in the impl header. We could make this specific thing work by moving T: Display to the impl header.

But it's questionable whether we should support generic functions at all. impl Trait in argument position could be complicated. We would have to transform impl Trait generics into standard generics. If we would only support standard generics, that would... be kinda sad.

Since we never claimed to support generics for Fn* types, I won't classify this as bug. But it would be nice to support in the future.

[KodrAus]

Could we use a similar technique to finding a reasonable name for generic types that we do with find_suitable_param_names?