better debuggability for interpreted procedures

better debuggability for interpreted procedures

* libguile/procprop.c (scm_set_procedure_minimum_arity_x): New
  function, allows a user to override a function's arity.
  (scm_i_procedure_arity): Look up in the overrides table first.

* libguile/procprop.h: Add scm_set_procedure_minimum_arity_x.

* module/ice-9/eval.scm (primitive-eval): Override arity of "general
  closures".

* test-suite/tests/procprop.test ("procedure-arity"): Add tests.

Based on a patch from Stefan Israelsson Tampe.  Test based on work by
Patrick Bernaud.