Addressed devices

The adev unit provides a simple but powerful abstraction over
C@ and C!: A@ and A!. These work the same way as C@ and C! (but
for performance reasons, aren't used in core words), but are
indirect calls.

Upon initialization, the default to C@ and C!, but can be set
to any word through A@* and A!*.

On top of that, it provides a few core-like words such as
AMOVE.

Let's demonstrate its use through a toy example:

                                                        (cont.)
