* alloc.c: Integer signedness and overflow fixes.

* alloc.c: Integer signedness and overflow fixes.

Do not impose an arbitrary 32-bit limit on malloc sizes when debugging.
(__malloc_size_t): Default to size_t, not to int.
(pure_size, pure_bytes_used_before_overflow, stack_copy_size)
(Fgarbage_collect, mark_object_loop_halt, mark_object):
Prefer ptrdiff_t to size_t when either would do, as we prefer
signed integers.
(XMALLOC_OVERRUN_CHECK_OVERHEAD): New macro.
(xmalloc_overrun_check_header, xmalloc_overrun_check_trailer):
Now const.  Initialize with values that are in range even if char
is signed.
(XMALLOC_PUT_SIZE, XMALLOC_GET_SIZE): Remove, replacing with ...
(xmalloc_put_size, xmalloc_get_size): New functions.  All uses changed.
These functions do the right thing with sizes > 2**32.
(check_depth): Now ptrdiff_t, not int.
(overrun_check_malloc, overrun_check_realloc, overrun_check_free):
Adjust to new way of storing sizes.  Check for size overflow bugs
in rest of code.
(STRING_BYTES_MAX): Adjust to new overheads.  The old code was
slightly wrong anyway, as it missed one instance of
XMALLOC_OVERRUN_CHECK_OVERHEAD.
(refill_memory_reserve): Omit needless cast to size_t.
(mark_object_loop_halt): Mark as externally visible.