parser: factor out function that parses right-hand side of prefix/infix ops These two had common pattern. The significant difference was just a result expression: prefix: (op-name, rhs) infix: (op-name, lhs, rhs)

parser: remove unused parameter 'pos' from _match() This backs out 9d1cf337a78d. The issue spotted by that changeset was addressed earlier by d4cafcb63f77.