install faster perl6 version

This version is twice as fast and cheats more like the nqp version.

perl6/rc-self-describing-numbers

@@ -1,8 +1,14 @@
 sub is_selfdesc($n) {
-    my @b = 0 xx $n.chars;
-    @b[$_]++ for my @a = $n.comb;
-    @b[$_] //= 0 for ^@b;
-    @a eq @b;
+    my $s = $n.Str;
+    my $chars = $s.chars;
+    my @a = +«$s.comb;
+    my @b;
+    for @a -> $i {
+	return False if $i >= $chars;
+	++@b[$i];
+    }
+    @b[$_] //= 0 for ^$chars;
+    @a eqv @b;
 }
 
 # check all numbers from 0 to SCALE