Removed some commented stuff in the base grammar.

Added aggregation operations (sum and avg) to the extended grammar to operate on looped/repeated items.
Renamed 'loop' to 'repeat'.
Replaced usage of Tuple to an own implementation: IPair/Pair. The Interface has a covariant type parameter giving the ability to put sub-types into a super-type pair, which was not possible with tuples.
Changed form building code A LOT! Can be made nicer by separating evaluation but this would require separating factories as well.
Added sum and avg to the styled printer thing.
Changed TypeChecker error handling.