Switch to unsigned types for holding Unicode code points

Switch to unsigned types for holding Unicode code points

We were only using negative numbers for the "no Unicode mapping"
constant, and even there it was mostly cast to unsigned before we used
it (because it needed to sort last).  So just using an unsigned type
throughout seems to be a bit cleaner.  It will also make me less dubious
about packing extra information in when I want to support variation
selectors.