Reduce call-with-values to let for singly-valued producers

Reduce call-with-values to let for singly-valued producers

* module/language/tree-il/peval.scm (singly-valued-expression?): Add
  support for conditionals.  In the future we should add more
  expressions here.
  (peval): Don't inline values into the body of a dynwind, as that could
  cause the consumer to run in the wrong dynamic context.
  If the producer is singly-valued and the consumer just has a rest arg,
  reduce to "let" and cons up a list in the consumer.  This may reduce
  further.

* test-suite/tests/peval.test ("partial evaluation"): Add a test.