Reuse bstack and cstack when restarting a cursor

While cursors and their match extents are immutable and we must make a
new one when backtracking, it seems there's no situation where we will
backtrack twice into the exact same cursor. Assuming this is correct (I
can't think of a case, the tests don't bring up a case), then it is safe
to pass ownership of them from one cursor to another.

In a case like /a (.+) c/ on a string like "abc" ~ "d" x 8000, this
means we do not need to repeatedly clone a huge bstack one per char that
we move back.

Helps with rakudo/rakudo#4236, where the golf
mentioned above uses vastly less memory and runs over 6 times faster.

src/QRegex/Cursor.nqp

@@ -431,8 +431,10 @@ role NQPMatchRole is export {
             nqp::bindattr_i($new, $?CLASS, '$!from', $!from);
             nqp::bindattr_i($new, $?CLASS, '$!pos', $!pos);
             nqp::bindattr_i($new, $?CLASS, '$!to', -1);
-            nqp::bindattr($new, $?CLASS, '$!cstack', nqp::clone($!cstack)) if $!cstack;
-            nqp::bindattr($new, $?CLASS, '$!bstack', nqp::clone($!bstack));
+            nqp::bindattr($new, $?CLASS, '$!cstack', $!cstack);
+            nqp::bindattr($new, $?CLASS, '$!bstack', $!bstack);
+            nqp::bindattr(self, $?CLASS, '$!bstack', nqp::null());
+            nqp::bindattr(self, $?CLASS, '$!cstack', nqp::null());
             $new
         }
         else {