Fixing a FIXME turned out to be pretty involved. I added an io function
that returns a unique boxed string (for the contents of a file) rather than
a string, and went from there. Also made the src field of codemap a unique
boxed string. This doesn't seem to make that much difference in amount of
allocation according to valgrind (disappointingly), but I also had to introduce
a copy somewhere else pending a new snapshot, so maybe that's it.
Use functions instead of a dynamically created table to determine
operator precedence. Gets rid of a FIXME in syntax::parse::prec.
Change precedences from int to uint while we're at it, since
don't use negative precedences.
