allow case-lambda expressions with no clauses

allow case-lambda expressions with no clauses

* module/ice-9/psyntax-pp.scm:
* module/ice-9/psyntax.scm (case-lambda, case-lambda*): Allow 0
  clauses.

* module/language/scheme/decompile-tree-il.scm (do-decompile):
  (choose-output-names):
* module/language/tree-il.scm (unparse-tree-il):
  (tree-il-fold, post-order!, pre-order!):
* module/language/tree-il/effects.scm (make-effects-analyzer):
* module/language/tree-il/cse.scm (cse):
* module/language/tree-il/debug.scm (verify-tree-il):
* module/language/tree-il/peval.scm (peval): Allow for lambda-body to be
  #f.

* libguile/memoize.c (memoize):
* module/language/tree-il/canonicalize.scm (canonicalize!): Give a body
  to empty case-lambda before evaluating it or compiling it,
  respectively.

* test-suite/tests/optargs.test ("case-lambda", "case-lambda*"): Add
  tests.