Implemented Async.Array.mapi2.
Re-implemented some of the Async.Array functions to use a recursive style (like Async.List) which minimizes memory overhead.
Implemented some new Async.Array functions and stubbed unit tests for them.
Implemented some unit tests for Async.Array functions.
Implemented Array.foldPairwise and Array.foldPairwiseBack and stubbed unit tests for them.