Named impl with Implementation Selection Variant

viruscamp:

1. Named-impl must be forbidden for Hash, Ord, PartialOrd, Eq, PartialEq, etc. at least when a default implementation exists.

#[disable_named_impl(exist_default)]
pub trait Hash {}

Objectionable. I might want to sort tuples (u32, u32) by their second argument, or use another custom comparator, while there is a default PartialOrd implementation.