Make List.roll(*) about 1.5x faster

- also eliminate R:I.Roller: there are  no clients for that anymore

src/core/List.pm

@@ -1005,8 +1005,10 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
           self.is-lazy,
           X::Cannot::Lazy.new(:action('.roll from')).throw,
           Seq.new(nqp::if(
-            self.elems,
-            Rakudo::Iterator.Roller(self),
+            (my $elems := self.elems),
+            Rakudo::Iterator.Callable( {
+                nqp::atpos($!reified, $elems.rand.floor)
+            }, True ),
             Rakudo::Iterator.Empty
           ))
         )

src/core/Rakudo/Iterator.pm

@@ -2681,13 +2681,6 @@ class Rakudo::Iterator {
         }.new(&body,&cond)
     }
 
-    # Return a lazy iterator that keeps calling .roll on the given object.
-    # Basically the functionality of List.roll(*), but could be on any
-    # object that has a .roll method.
-    method Roller(\source) {
-        Rakudo::Iterator.Callable( { source.roll }, True )
-    }
-
     # Return an iterator that rotorizes the given iterator with the
     # given cycle.  If the cycle is a Cool, then it is assumed to
     # be a single Int value to R:It.Batch with.  Otherwise it is