Fixing issue 9

[notalfredo]

Fixes #9

I had a different approach let me know what you think. For levenshtein distance I converted the matrix computations functions as a helper function. Then diff and distance call this helper function. This way we don't have any dupe code. Hamming works similar

[neoncitylights]

Thanks for the PR! Just a few comments.

If you look at the files tab, it shows the branch is outdated because it also shows splitting out the lib.rs into 4 different files (also see screenshot). Unfortunately it's not possible through GitHub itself to make a PR dependent on another PR, so this will have to wait until #13 is gone through, since it'll be a little difficult reviewing this change.

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Can you make the commit message title a little more specific? While it technically explain what's being fixed, it's not possible to understand what's going on purely from the title without visiting a link.

To illustrate, if the commit history was like:

 - fix: Fix issue #9 
 - feat: Implement feature #120
 - docs: Fix docs #19

It'd be a bit difficult to track things down, or understand how the software was changing over time, like e.g if there was a regression (a bug was fixed at one point, but then accidentally re-introduced in the future). Since #9 says there's duplicate code, the title should be something along the lines of like:

chore: Deduplicate code between diff() and distance()

<commit message body explains how code was deduplicated>