HOTFIX: Remove locking in API GC_get_bytes_since_gc and friends.

HOTFIX: Remove locking in API GC_get_bytes_since_gc and friends.
Add GC_get_heap_usage_safe() to API as a thread-safe alternative to
GC_get_bytes_since_gc and friends.
Remove newly-added lock-free GC_get_heap_size_inner and
GC_get_free_bytes_inner from API.

* include/gc.h (GC_get_heap_size, GC_get_free_bytes,
GC_get_unmapped_bytes, GC_get_bytes_since_gc, GC_get_total_bytes):
Refine comment.
* include/gc.h (GC_HAVE_GET_HEAP_USAGE_SAFE): New macro.
* include/gc.h (GC_get_heap_usage_safe): New API function.
* misc.c (GC_get_heap_usage_safe): Likewise.
* include/gc_mark.h (GC_get_heap_size_inner, GC_get_free_bytes_inner):
Remove API function.
* misc.c (GC_get_heap_size_inner, GC_get_free_bytes_inner): Likewise.
* misc.c (GC_get_heap_size): Add comment.
* misc.c (GC_get_heap_size, GC_get_free_bytes, GC_get_unmapped_bytes,
GC_get_bytes_since_gc, GC_get_total_bytes): Remove locking.
* misc.c (GC_get_unmapped_bytes): Do not test USE_MUNMAP (not needed
after locking removal).
* misc.c (GC_get_bytes_since_gc, GC_get_total_bytes): Cast result to
size_t.