More temporary hacking around the to-be-renamed `permutations' problem.
Make `in-producer' treat any number of values uniformly.

The previous version was broken in its inconsistent treatment of
multiple values, which was motivated by an `in-generator' use
case.  (`in-generator' should be fixed too now.)  The new version is
much simpler since there's no need for three almost-exact copies of the
same code.