coloncircumfix was stealing :() pseudo-sigs

(This was due to the new longest-literal tiebreaking rule.)

src/Perl6/Grammar.nqp

@@ -1877,7 +1877,7 @@ grammar Perl6::Grammar is HLL::Grammar does STD {
     token coloncircumfix($front) {
         [
         | '<>' <.worry("Pair with <> really means an empty list, not null string; use :$front" ~ "('') to represent the null string,\n  or :$front" ~ "() to represent the empty list more accurately")>
-        | <circumfix>
+        | {} <circumfix>
         ]
     }