psyntax supports multiple docstrings, merging lambda-case metadata

psyntax supports multiple docstrings, merging lambda-case metadata

* module/ice-9/psyntax.scm (build-simple-lambda, build-case-lambda):
  Don't take a docstring, take a metadata alist. Don't bother annotating
  interpreted expressions with docstrings.
  (chi-simple-lambda): Take already-parsed metadata.
  (lambda): Parse out multiple docstrings as separate "documentation"
  entries in the "meta".
  (chi-lambda-case): Merge multiple "meta" entries among the various
  cases.
  (lambda*, case-lambda, case-lambda*): Receive "meta" clauses, not
  docstrings.

* module/ice-9/psyntax-pp.scm: Regenerated.