Add base class for implementations and get_handle function

All implementation objects are pointed by handle. Of course handles contains
reference to its implementations. But it turns out, that such connection is
required also in reverse direction. When some implementation object returns
some another implementation object, then user must receive handle of this
object.

To solve above problem implementation objects contains reference to its
handlers. Handle is private and can be accessed with `get_handle` helper
function. For all implementation object is provided `Implementation` base
class.