Fix "Method 'is-lazy' not found" regression in META_CROSS

Commit 7bc1a99 replaced calls to
.infinite by the new .is-lazy. META_CROSS may call .is-lazy now on
simple scalars like Int. Mu provided a method infinite for such cases.
Replace that by a method is-lazy that defaults to False.

Thus the migration from infinite to is-lazy is complete.

src/core/Mu.pm

@@ -104,8 +104,8 @@ Please refactor this code using the new Iterator / Seq interface.
     }
 
     # XXX GLR does this survive?
-    proto method infinite (|) { * }
-    multi method infinite(Mu:) { Nil }
+    proto method is-lazy (|) { * }
+    multi method is-lazy(Mu:) { False }
 
     method CREATE() {
         nqp::create(self)