parser: separate actions for primary expression and prefix operator This will allow us to define both a primary expression, ":", and a prefix operator, ":y". The ambiguity will be resolved by the next patch. Prefix actions in elements table are adjusted as follows: original prefix primary prefix ----------------- -------- ----------------- ("group", 1, ")") -> n/a ("group", 1, ")") ("negate", 19) -> n/a ("negate", 19) ("symbol",) -> "symbol" n/a