fix unit tests
Merge pull request #4 from enricosada/fix_tests

Fixed the implementations of IterateBack in IntSet/IntMap/LongSet/LongMap so they're self-recursive instead of incorrectly calling Iterate.
Fixed a few failing unit tests for String and Substring.
Fixed PatriciaMap64.IterateBack so it's self-recursive as expected (instead of calling Iterate).
Implemented some tests for the ExtCore.Operators module.
Implemented Seq.sample.