Fix accumulatingLeft, add tests

[mossheim]

• Add documentation
• Don't do a second reverse, so that the output behaves as expected
• Add tests to confirm new behavior

@jsbean, is this output really what's expected? I would have expected [1,2,3].accumulatingRight to be [1,3,6], and leftwise [3,5,6].

[codecov-io]

Codecov Report

Merging #87 into master will increase coverage by 0.31%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   81.97%   82.28%   +0.31%     
==========================================
  Files          47       47              
  Lines        1187     1197      +10     
==========================================
+ Hits          973      985      +12     
+ Misses        214      212       -2

Impacted Files	Coverage Δ
ArithmeticToolsTests/CumulativeTests.swift	100% <100%> (ø)	:arrow_up:
...meticTools/BidirectionalCollectionExtensions.swift	88.23% <100%> (+5.88%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 9ba8754...29b9554. Read the comment docs.

[jsbean]

I'm gonna check where I am using that. There is a good chance both are wanted, with a horrible naming conflict in the middle :).

[jsbean]

It looks like it was needed at some point, though it looks like no longer. Therefore, I agree.