Provide an extension point for checking the return type in Fn concepts

[danakj]

Right now we have Anything and NonVoid as in sus::fn::FnMut<Anything()> but it's very common to constrain on other return types like "returns an Option". So we should be able to write a type IsOption that the FnMut concept can test against without saying it is that type, but that it accepts that type.

Then replace a ton of invoke_result_t && IsOption::value constraints and others like it.