Operator precedence parsing

Once again, we've borrowed a bit of Perl 6-like syntax for this:

proto infix:+ is precedence('=') { ... }
proto infix:- is equiv('infix:+') { ... }

proto infix:* is tighter('infix:+') { ... }
proto infix:/ is equiv('infix:*') { ... }

This defines the four basic arithmetic operators.

These definitions can go in the same grammar input file that is provided to pgc.pir . It then creates the code needed to initialize an operator table.