Worker/wrapper: Refactor CPR WW to work for nested CPR (#18174)

In another small step towards bringing a manageable variant of Nested
CPR into GHC, this patch refactors worker/wrapper to be able to exploit
Nested CPR signatures. See the new Note [Worker/wrapper for CPR].

The nested code path is currently not triggered, though, because all
signatures that we annotate are still flat. So purely a refactoring.
I am very confident that it works, because I ripped it off !1866 95%
unchanged.

A few test case outputs changed, but only it's auxiliary names only.
I also added test cases for #18109 and #18401.

There's a 2.6% metric increase in T13056 after a rebase, caused by an
additional Simplifier run. It appears b1d0b9c saw a similar additional
iteration. I think it's just a fluke.

Metric Increase:
    T13056