Use trait, not impl, definition for X_as_Trait functions

Use trait, not impl, definition for X_as_Trait functions

When we map a trait impl block, we historically didn't know how to
resolve the types in the trait context, so we just used the impl
context definitions and assumed the types were the same. Sadly, if
we have an associated type that is bounded by a trait, we need to
use the trait bound to figure out what to write for the X_as_Trait
method declarations.

Since we now have the ability to generate full trait-context type
resolvers, we do so here.