Deduplicate longest common subsequence logic

[goretkin]

This is pretty much purely an aesthetic suggestion to deduplicate the logic (prompted because I had to change == -> isequal in two places in https://github.com/ssfrr/DeepDiffs.jl/pull/14 . This PR depends on that PR.)

I feel like the changes lie on a spectrum. For that reason, I broke the change out into a million commits. If the code at the last commit doesn't look so great, perhaps one just before is better. Also, each commit mostly represents a straightforward transformation.