src/pset-parse.lisp: Replace `dispatch' by some more elementary functions.

src/pset-parse.lisp: Replace `dispatch' by some more elementary functions.

This turns out to be an overall saving in terms of lines of code, as
well as being more versatile.  The price is that we've lost the specific
per-operator type mismatch error, but I think that's worth paying.

Somewhat sneakily, I've also fiddled with the operator precedence
numbers, so as to leave more space for other operators, though the
relative precedences are unchanged.