Improve test coverage

Introduces a breaking change in the HashBytes function, and a potential
breaking change in the NewFixedBlocksize function.
Remove provably redundant clause

Because the score function requires a common substring of length at
least 7, score will only be larger than 100 when editDistance d
satisfies the equation

	   64d
	 ------- = 64
	 14 + d

for positive d.  This will never happen.  The if clause was only present
because it was present in the original code.