system xref maintains source mapping for nested procedures too

system xref maintains source mapping for nested procedures too

* module/system/xref.scm (*closure-sources-db*): New global, like
  *sources-db* but for nested procedures. It's a separate map because
  these procs need to be treated differently in trap handlers -- you
  match on the bytecode, not on the program object.
  (add-source, forget-source): Take the db as an argument (the normal db
  or the closures db).
  (add-sources, forget-sources): Record sources for nested procedures to
  in *closures-db*.
  (untaint-sources, ensure-sources-db): Adapt for new closures db.
  (lookup-source-procedures): Factored out.
  (source-closures): New exported procedure, returns closures at the
  given source location.