* gc-malloc.c (scm_calloc): try to use calloc() before calling
scm_realloc().
* gc-segment.c (scm_i_initialize_heap_segment_data): remove card
init loop; handle this from scm_init_card_freelist()
* gc-card.c (scm_init_card_freelist): init bit vector here.
* gc-card.c ("sweep_card"): set scm_gc_running while sweeping.
* gc.c (scm_i_expensive_validation_check): separate expensive
validation checks from cheap ones.
