peval: simpler and more precise treatment of mutability

peval: simpler and more precise treatment of mutability

* module/language/tree-il/optimize.scm (peval): The old approach of
  optimistically producing constants and then de-constifying them at
  their uses was not only cumbersome but incorrect: it both failed to
  preserve identity in some cases and failed to retain immutable
  constant values.  Instead, now we only produce constants if they
  really are constant and immutable.  The constant folder has to have a
  few more algebraic cases to be as effective as it was, to destructure
  (car (cons _ _)) appropriately.  On the plus side, now constructors
  and deconstructors can handle impure cases more generally.

* test-suite/tests/tree-il.test ("partial evaluation"): Add constructor
  and destructuring tests.  Adapt other tests to new expectations.