But.. we don't really use it for anything right now.

The idea here is that an incoming query may refer to some universes,
and they query response may contain fresh universes that go beyond
those. When we instantiate the query response in the caller's scope,
therefore, we map those new universes into fresh universes for the
caller.

In particular, we don't want to preserve the universes for the `'_`
variables that appear in there. And we don't expect to find any
placeholders, which justifies this as harmless.

(In particular, if you have a query like `Foo(!1, !2, ?3)`, then you
care about the universe of `?3`, since it may control whether `?3 =
!1` and `?3 = !2` is a valid answer. But without any placeholders, we
don't really care: any placeholders that would appear in the output
must therefore come from some fresh universe anyway.)

Required for test expect-fn-supply-fn.rs to pass; otherwise we have
unconstrained inference variables that get inferred to `'empty`.

Account for incompatible universes and higher-ranked subtyping.