Operator precedence parsing

Once again, pgc.pir has borrowed a bit of Perl 6-like syntax:

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, and generates code to initialize an operator precedence parser.

These definitions can go in the same grammar input file that defines other rules.