add program-sources-pre-retire to core and define frame-next-source

add program-sources-pre-retire to core and define frame-next-source

* libguile/programs.h:
* libguile/programs.c (scm_program_source): Add an optional arg, the
  sources table to traverse. Defaults to the result of
  scm_program_sources.

* module/system/vm/program.scm (program-sources-pre-retire): Move
  definition here from (system vm traps), and export.

* module/system/vm/traps.scm: Adapt.

* module/system/vm/frame.scm (frame-next-source): New exported binding,
  returns the source line corresponding to the next instruction instead
  of the previous instruction.