Make Mu.iterator use R:I.OneValue

The comment was out of date.  Needs a fix in roast where defined Mu value
is improperly tested for.

src/core.c/Mu.pm6

@@ -50,14 +50,7 @@ my class Mu { # declared in BOOTSTRAP
     }
 
     proto method iterator(|) {*}
-    multi method iterator(Mu:) {
-        my $buf := nqp::create(IterationBuffer);
-        $buf.push(Mu);
-        # note: cannot use R:I.OneValue, as that doesn't (and shouldn't)
-        # take Mu for the value to produce, as Mu is used to indicate
-        # exhaustion.
-        Rakudo::Iterator.ReifiedList($buf)
-    }
+    multi method iterator(Mu:) { Rakudo::Iterator.OneValue(self) }
 
     proto method split(|) {*}