TemplateParameters' methods should just return a Vec, not an Option<Vec> #960
Description
I thought that the Option would be useful, but in practice we do pretty much ignore that it is an option and do things like params.unwrap_or(vec![]). And then in other places when the result is Some we assume that the contained Vec is not empty, when that isn't always the case.
It will be much simpler to just return a Vec that is either empty or not.
And also usize instead of Option<usize> for num_self_template_params.
The trait's comment will need to be updated as well.