Fix incorrect slicing #3

Slices are currently incorrect. Right now, for example:

Fr = Range(0, 1, 2, 3);
print(Fr); // "Any|[0, 1, 2, 3]"
print(Fr[0:-1]); // "Any|[0, 1, 2]"
print(Fr[]); // does not interpret

So thus the negative slicing Fr[0:-1] is incorrect, and needs to be fixed. It should print out a slice that is identical to Fr.
Fr[] should be valid, and it should print out an explicit copy of Fr. That is, they should 'point' to different memory addresses.

Also these tests (and more with slices) should be added to unittest.py