More concise reprs for Slice

[asmeurer]

Currently Slice always prints all three arguments, even if only one or two are needed to create it:

>>> Slice(None)
Slice(None, None, None)
>>> Slice(0, 10)
Slice(0, 10, None)

these could be printed more concisely as Slice(None) and Slice(0, 10).

We should also fix the printing of slice in Tuples so that something like Tuple(slice(None), slice(0, 10)) prints as such instead of Tuple(slice(None, None, None), slice(0, 10, None)).

(note, printing slices using : notation is a separate issue. This would be useful, but is only valid inside of an index expression so it should be done in a separate method)