Parse "1 2 +" as an expr
 Try 'number'
  Parse 'number', left with "2 +"
  Parse 'end', PANIC!
 Try 'expr unary'
  Parse 'number', left with "2 +"
  Parse 'unary', PANIC!
 Try 'expr binary'
  Parse 'number', left with "2 +"
  Parse 'number', left with "+"
  Parse 'binary', left with ""
  Parse 'end', done!