Break out of the merge loop if the left part is done, otherwise we would compare witht he wrong thing.
Just realized there is no need to copy the remainder of the right side subarray. You can just decrement the endpoint saving on come copying.
Read the PAD_SIZE form the environment.

make tools still used the definition from the Makefile
Added import for recursive_mergesort
Removed setting of element size from this, since it doesn't do the building anymore
Change the PAD_SIZE to 7 to avoid the struct to be exactly 8 bytes which gives the stdlib sorts an advantage (I think)
Removed inplace_mergesort as it is horribly slow
Added benchmark SVG for mergesorts