Refactor deployer and dispenser

Merge implementations into a single "wielder" mechanism.  Clean up the
code a bit.  Fix get_player_control_bits for the dispenser (it wasn't
indicating the sneak setting).  Fix drops for dispenser (dispenser_on was
dropping as itself).  Show item image, as well as name, at head of form.
Refactor node breaker

Merge node breaker into the generic wielder mechanism.  Center the wield
inventory in all wielders' formspecs.  Implement full auto-upgrade of
legacy node breakers, occurring upon use or punching of the node breaker.
Make node breaker respect on_dig hooks.