Removing 'rm testdata/' form 'clean' target as that is really annoying.
Added the benchmark that compares all sorts.

Except that I had forgotten timsort was still broken in Backwards Gallop Mode.
Added assert()s for gallop mode index finding as going backwards is fiddly.

Also renamed the chunk length to lengths instead of sizes since size indicates
number of bytes and length number of elements.
Unconfused left/right pointers for the assertions and verbosity.