Make a faster List.pick(*,:eager) version

Well, that was the idea.  Turns out the eager version is about 20x *slower*.
Added here for further research.

What has been ruled out:
- anything inside the loop
  That part of the code is mostly the same as the non-eager version.  If
  the code inside the while loop is reduced to just the $elems = $elems - 1,
  even then the eager variant is about 15x slower.
- something in the MMD dispatch
  If the code inside the "eager" variant is the same as the non-eager variant,
  there is no difference in execution noticeable either.
- not SPESH related
  With MVM_SPESH_DISABLE=1 the difference in execution is even larger: the
  optimizer did its best!

What *is* different?
- the number of frames in a profile
  example code: my @A = ^100; for ^500 { @a.pick(*) } # or *, :eager
  non-eager / eager: 10618 / 176614 frames
  So the difference in number of frames is in same order of magnitude as the
  difference in CPU.

So the question is really: why does the eager version create so many frames?

src/core/List.pm

@@ -181,6 +181,27 @@ my class List does Positional { # declared in BOOTSTRAP
         my $elems = self.elems;
         $elems ?? self.AT-POS($elems.rand.floor) !! Nil;
     }
+    multi method pick(Whatever, :$eager!) {
+        return self.pick(*) if !$eager;
+
+        fail X::Cannot::Infinite.new(:action<.pick from>) if self.infinite;
+
+        my Int $elems = self.elems;
+        return unless $elems;
+
+        my Mu $picked := nqp::clone($!items);
+        my Int $i;
+        my Mu $val;
+        while $elems {
+            $i     = nqp::rand_I(nqp::decont($elems), Int);
+            $elems = $elems - 1;
+            # switch them
+            $val  := nqp::atpos($picked,nqp::unbox_i($i));
+            nqp::bindpos($picked,nqp::unbox_i($i),nqp::atpos($picked,nqp::unbox_i($elems)));
+            nqp::bindpos($picked,nqp::unbox_i($elems),$val);
+        }
+        nqp::p6parcel($picked,Any);
+    }
     multi method pick(Whatever) {
         fail X::Cannot::Infinite.new(:action<.pick from>) if self.infinite;