Operator precedence parsing

Defining operator tokens and precedence:

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.