The values in `IdentifiedFacetType` may contain `IdentifiedWitness`
instructions, and those insts need to be either converted to
`LookupImplWitness` and evaluated as such, or substituted by the user of
the `IdentifiedFacetType` with witnesses of its own.
This changes the API so that the user must go through a method doing one
of those two things to get to the underlying values in the
`IdentifiedFacetType`, unless they explcitly request to bypass them.
Then we split extend impls requirements out into their own set so that
the user can determine what is being extended _after_ evaluation or
substitution of `IdentifiedWitness` insts. This is critical because we
can not dedupe SpecificInterfaces until after that takes places, as
demonstrated by the test fixed by this PR.
Dana Jansens
ac31fd1985