Rewrite expression parser as a Pratt parser

[nlfiedler]

See http://journal.stuffwithstuff.com/2011/03/19/pratt-parsers-expression-parsing-made-easy/ for a discussion regarding a Pratt parser for parsing expressions that have infix, prefix, and postfix operators. Especially tantalizing is the treatment of ( as both a prefix operator (for grouping) and an infix operator (for function calls). Would make the existing parser much simpler and easier to debug.