Removed automatic memory merge from physics.

If a merge is desired then the proposal function can propose that
:memory be (merge (:memory p) ...). The automatic merging would have
made forgetting difficult.
Reverted memory merge; added state-based darters.

Determined that requiring full memory merging in proposal functions is
too onerous; arbitrate-proposals will now (again) merge proposed memory
map with previous memory map; values can be easily forgotten, although
not keys. Darters demonstrate agents with state-based control
architecture. The pucks.worlds.ai.world5 world demonstrates darters.
Version bump.