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.
<dynwind> no longer has "pre" or "post" fields

* module/language/tree-il.scm (<tree-il>): Remove pre and post fields
  from <dynwind>.  A dynwind now assumes that in normal entry and exit,
  that the code runs the winders and unwinders using <seq> and
  <let-values> and such things.
  (parse-tree-il, unparse-tree-il, make-tree-il-folder, pre-post-order):
  Adapt <dynwind> users.

* module/language/tree-il/analyze.scm (analyze-lexicals):
* module/language/tree-il/compile-glil.scm (flatten-lambda-case):
* module/language/tree-il/cse.scm (cse):
* module/language/tree-il/debug.scm (verify-tree-il):
* module/language/tree-il/effects.scm (make-effects-analyzer): Adapt.

* module/language/tree-il/peval.scm (peval):
* module/language/tree-il/primitives.scm (*primitive-expand-table*):
  Produce tree-il that calls the winder and unwinder.  Recognize
  singly-valued dynamic-wind expressions.

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