* gc.c (scm_gc_stats): add more obscure stats, such as: mark time,

* gc.c (scm_gc_stats): add more obscure stats, such as: mark time,
sweep time, total marked cells, total swept cells, and number of
times GC was invoked.
(gc_start_stats): renamed from scm_gc_start, made static, taught
to init the new stats.
(gc_end_stats): renamed from scm_gc_end, made static, taught to
calculate the new stats.
(scm_igc): don't call gc_start_stats unless we are sure that we
are indeed going to collect.  also, added some timekeeping between
the mark and sweep phases.
(scm_gc_sweep): count number of cells we sweep as we go.

* gc.h: removed prototypes for scm_gc_{start,end}.