Rename 'Insert' to 'TryAdd' and 'InsertOverwrite' to 'Add' to match the names of the same functions for the F# Map.
Implemented some unit tests for IntSet.
Stubbed out some tests for the BitOps module used by the IntSet and IntMap implementations.
Implemented tests for IntSet.ofArray.
Correctness fixes for IntMap (the Add and TryAdd methods were swapped accidentally).
Implemented some optimizations for IntSet and IntMap which avoid creating unnecessary identical objects.
Changed instance members of PatriciaMap and PatriciaSet to static members and enabled the 'UseNullAsTrueValue' optimization for the Empty case.