Indenting test code/commands for preformatted text markup
Fix for rollsum16 not producing as many different hashes as it should

FULL_BYTES_16 was not 15 1s in binary. Hash now produces significantly better hash values.
This also improves the performance of diffing, reducing MD5 computation and comparison.

Should do some more tests to ensure that the rollsum is producing exactly the expected values.
patch reports the number of weak checksums in the structure.
Updating README with performance information