3040: Rework value parameter parsing r=matklad a=tobz1000
Fixes#2847.
- `Fn__(...)` parameters with idents/patterns no longer parse
- Trait function parameters with arbitrary patterns parse
- Trait function parameters without idents/patterns no longer parse
- `fn(...)` parameters no longer parse with patterns other than a single ident
__Question__: The pre-existing test `param_list_opt_patterns` has been kept as-is, although the name no longer makes sense (it's testing `Fn__(...)` params, which aren't allowed patterns any more). What would be best to do about this?
Co-authored-by: Toby Dimmick <tobydimmick@pm.me>
6d6a995e09