Parse + binary operator as a call expression

It made **some** sense initially to parse this as a separate expression,
but we can save some time later by recognizing that + is really just a method
like any other, and that it can be implemented differently for different
classes (or even monkey patched)
Remove set -e from build script

Failed if bin/ruby and bin/irb did not already exist. BUmmer.
Parse subtraction as a call expression
Rewrite multiplication as a call expression