Change vector size accounting to bytes for consistency

Count vector allocation sizes in bytes instead of sizeof(VECREC) to
ensure that the values are consistent with all other allocation size
statistics.
Avoid double-counting of null-length vector allocations

Ensure that null-length vector allocations are counted only as such
instead of double-counting them as a noncons allocation too.
Fix an off-by-one accounting error in the number of duplicated elements

The number of duplicated elements was incremented by 1 on every entry
of duplicate1() and once again by the number of elements whenever an
atomic vector was copied, resulting in an off-by-one error for these
vectors.

Fixed by adjusting the second increment to compensate for the one at
the beginning of the function.