Implement Array.cow (for copy-on-write)

This is basically just to get it out there and may be revoked at any
time before the next release.

It basically creates a copy of Array with aliases to the values:

  my @A = ^10;
  my @b = @a.cow;   # values are shared
  @b[5] = 42;       # element #5 no longer shared

This as a result of a question at The Perl Conference:
  Can you do "Copy-On-Write" in Perl 6?

I thought this could be useful.

src/core/Array.pm

@@ -40,6 +40,21 @@ my class Array { # declared in BOOTSTRAP
         }
     }
 
+    method cow() {
+        my int $elems = self.elems; # ensure reification
+        my $src  := nqp::getattr(self,List,'$!reified');
+        my $cow  := nqp::setelems(nqp::list,$elems);
+        my int $i = -1;
+
+        nqp::while(
+          nqp::islt_i(($i = nqp::add_i($i,1)),$elems),
+          nqp::bindpos($cow,$i,nqp::p6scalarfromdesc($!descriptor) =
+            nqp::decont(nqp::atpos($src,$i)))
+        );
+
+        nqp::p6bindattrinvres(nqp::create(Array),List,'$!reified',$cow)
+    }
+
     method iterator(Array:D:) {
 
         # something to iterate over in the future