Reorganise code: move Trit and Trits into a submodule.
Implement the UTF-6t text encoding scheme.

This required changing one of the earlier rules I made about the Trits
class; empty Trits sequences were forbidden, but that just doesn't make
sense when dealing with text.

It also required me to go ahead and implement an unsigned integer
encoding scheme so that I could pack those unicode code points into a
sequence.

So we now have the 'integer' and 'character' submodules.  The integer
classes are Int and UInt, the only character class is UTF6t.

Next I would like to add some useful constructors to UTF6t, and
arithmetic operations to Int and UInt.