Named impls

[Ixrec]

Sometimes it's suggested that impls could have names, such that they have to be explicitly imported by name before they have any effect.

This is not a viable solution for orphan rule conflicts because it's unsound unless it's pure sugar, but that's not at all obvious to most people so it's worth noting that explicitly. In fact, Rust used to have named impls but removed them long before 1.0.