Cleans IBS a bit up
Fixes incorrect implementation of span in IBS, while using a new caching function to speed up things.
Adds some extra contracts on IIntervalCollection on Span, Add and Remove
Removes redundant parantheses
Adds Pure to LowestLow and HighestHigh
Optimizes several parts of DIT
Finishes (95%) code contracts including Span and LocalSpan.