Updated 20IntHaskellExercises.hs

made some solutions a bit more elegant, finished the last exercises
Modified MergeSorts.hs

Removed the vector merge sort - it turned out that it wasn't an error
along the way.  Algorithm was just that slow.
Modified Arithmetic.hs

Added extra functionality, made the type declarations nicer looking
Modified ListOps.hs

Made type declarations look nicer, made it export Data.List
Modified State.hs

Made the main function better