attempt to handle steady-state mallocations better

attempt to handle steady-state mallocations better

* libguile/gc-malloc.c (scm_realloc): Call the new
  scm_gc_register_allocation() here.  If we have to collect, do a
  GC_gcollect_and_unmap.

* libguile/gc.c (scm_gc_register_allocation): Add a routine to track
  steady-state mallocation, and cause gc to run if there is a high
  mallocation rate.
  (adjust_gc_frequency): Reset the bytes-until-GC countdown timer.