Implemented Array.exactlyOne, Array.Parallel.mapInPlace, and Array.Parallel.mapiInPlace functions.
Added full documentation comments for ListZipper.
Implemented several additional methods on the substring type; substring now also implements IEnumerable and IEnumerable<char>.
Fixed some of the search functions for substring so they return an index into the substring instead of an index into the underlying string.
Implemented some unit tests for the substring type and stubbed the others so they can be implemented in the future.