Fixed collapse(...level=1) case
Merge branch 'play' of https://github.com/sazlin/more-itertools into play
Fixed >>> tests for collapse3() to actually test collapse3() and not collapse2(). Tests pass.
Added collapse4, which is collapse 2 + the faster and cleaner check for iterables from collapse3, and with the outer collapse4()'s if statements broken up for the sake of perf. Benchmarks were updated and show collapse4 to be about 10% faster to use than collapse3.