programs can now get at their names, and print nicely

programs can now get at their names, and print nicely

* module/system/vm/frame.scm (frame-call-representation)
  (frame-program-name): Rename program-name to frame-program-name, and
  use the program-name if it is available.

* module/system/vm/program.scm (program-bindings): Return #f if there are
  no bindings.
  (program-name): New public procedure.
  (program-bindings-as-lambda-list, write-program): A more useful writer
  for programs.

* libguile/programs.c (scm_bootstrap_programs, program_print): Add a smob
  printer for programs, which dispatches to `write-program'.