Make sure Map.Hash works correctly

Before, any subsequent changes made to the Hash, would be happening to
the Map also.  This also makes Map.Hash 15x faster for an empty Map,
to the same speed for a Map with 20 keys, to slower for larger Maps.

src/core/Map.pm

@@ -12,9 +12,22 @@ my class Map does Iterable does Associative { # declared in BOOTSTRAP
 
     multi method Hash(Map:U:) { Hash }
     multi method Hash(Map:D:) {
-        my $hash := Hash.new;
-        nqp::bindattr($hash,Map,'$!storage',nqp::getattr(self,Map,'$!storage'));
-        $hash
+        if nqp::attrinited(self,Map,'$!storage') {
+            my $hash       := nqp::create(Hash);
+            my $storage    := nqp::bindattr($hash,Map,'$!storage',nqp::hash);
+            my $descriptor := nqp::null;
+            my $iter       := nqp::iterator(nqp::getattr(self,Map,'$!storage'));
+            while $iter {
+                my $tmp := nqp::shift($iter);
+                nqp::bindkey($storage,nqp::iterkey_s($tmp),
+                  nqp::p6scalarfromdesc($descriptor) =
+                    nqp::decont(nqp::iterval($tmp)));
+            }
+            $hash
+        }
+        else {
+            nqp::create(Hash)
+        }
     }
 
     multi method Bool(Map:D:) {