Make grep-index work when returning into an array

This feels like a work around for some obscure scoping issue.

src/core/Any.pm

@@ -130,7 +130,7 @@ my class Any { # declared in BOOTSTRAP
     }
     method grep-index(Mu $test) {
         my $index = -1;
-        self.map: { $index++; $index if $_ ~~ $test };
+        self.map: { $index++; +$index if $_ ~~ $test };
     }
     method first(Mu $test) is rw {
         self.map({ return $_ if $_ ~~ $test });