Named impl with Implementation Selection Variant

Tamschi:

This breaks adding or broadening a trait impl being SemVer-safe.

Actually, It's not SemVer-safe right now.

• lib A-1.0: struct A

• bin B:

  trait T { fn fn1(); } impl T for A { fn fn1() {} } A::fn1();

• lib A-1.1, add a new trait and impl, it breaks bin B

  struct A; trait T1 { fn fn1(); } impl T1 for A { fn fn1() {} }