More statprof state refactorings

More statprof state refactorings

* module/statprof.scm (existing-profiler-state): New helper, gets the
  profiler state or fails if there is no state.
  (sample-stack-procs, profile-signal-handler, count-call)
  (statprof-fold-call-data, statprof-proc-call-data)
  (statprof-call-data->stats, statprof-display)
  (statprof-display-anomolies, statprof-accumulated-time)
  (statprof-sample-count, statprof-fetch-stacks)
  (statprof-fetch-call-tree): Use the new helper.
  (statprof-active?): Don't create a state if there isn't one already.