reworked original 2.x version of xrange into range

changed from int to BigInteger to support ranges beyond 2^64
added applicable test from stdlib
added convenience methods to iptest/assert_util.py to mimic unittest
added slice operator
added == != < <= > >= operators
added __contains__ method
added count and index methods of collections.abc.Sequence
changed start, stop and step to be attributes
added __hash__
added support for objects implementing __index__