Wish list: new method Clear()

This change adds a new method Clear() to Tie::IxHash objects.

The same can be obtained with

      $ixh->Delete($ixh->Keys);

(whose performance should be very poor)

or

      $ixh->Splice(0, $ixh->Length);

(whose performance should be better, but it is still quite complicate for the simple task of Clear())

Both options also include calling two methods on the object.