Add substrings()

[bbayles]

Re: #227, I did like the "generate all windows" thing, so here's a thing to do that.

[pylang]

Neat implementation. To be clear, is the motivation to make a more efficient windowed?

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Since these items are contiguous elements, the formal name would be "substring" rather than subsequence. The classic name "substring" is terribly confusing imo, but here is the distinction:

• substring: contiguous elements "abc" -> "" "a" "b" "c" "ab" "bc" "abc" (similar to windowed)
• subsequence: contiguous/non-contiguous "abc" -> "" "a" "b" "c" "ab" "bc" "ac" "abc" (similar to powerset)

Notice like the haskell function, subsequences include the non-contiguous element "ac".

By contrast, "substring" is altogether a bad name. In particular, the tool can be mistakenly constrained to the str type when it should work on any iterable. However, I'm not sure what other name to call this tool aside from contigious_subsequences (not great).

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You may be seeking real problems to test this tool against. Consider the following:

• Leetcode challenge
• repeated substring
• SO longest common substring (corrected)

I find it helpful to have a reverse parameter is some of these cases.

[bbayles]

Ah, Wikipedia says:

The subsequence should not be confused with substring

Which is precisely what I did. I'll rename this.