There is a bit available in size (since it's always nonnegative)
but not in size_byte (since it ranges from -1 .. PTRDIFF_MAX
and is a ptrdiff_t, assuming a 32-bit host configured --with-wide-int
and assuming the memory-saving patch of Bug#9874).
This is in contrast with our current uses of mark bits (e.g.,
ARRAY_MARK_FLAG), which use bits that are otherwise unused, even if
a vector has its maximal size.
We can fairly easily get that bit back from size_byte by restricting
its range to (say) 0 .. PTRDIFF_MAX.
So this is a fairly minor glitch that can be fixed but is not yet
fixed in the current proposal.