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.