Negating a Trits object returns an object of the same type.

This way, when you do -Int(5) you get Int(-5), instead of Trits('-++').
Negating a UTF6t will most likely get you an exception, but that's
probably the right outcome for doing something that silly.
Add method to trim leading zeroes from a Trits sequence.

This isn't used anywhere else in the code yet, but I figure it's going
to come in handy pretty soon.
Fix some edge cases in integer division and add tests.

Also fill in the __floordiv__ and __mod__ methods, which just pull
individual values out of the __divmod__ result tuple but are still nice
to have in there.