Fixed incorrect memory stripping + more

Stripping of memory from sensed agents was done incorrectly; fixed this
and also strip neighbors, overlaps, and inventory. Also, partial
implementation of gates and keys
Tweaks to post-collision physics, more work on gates.
Gates, keys, and abbreviate-embedded-agents

Implemented gates and keys, and the transaction handling necessary to
support these. Also provided a abbreviate-embedded-agents function to
keep printing of agents reasonable, for example, when clicking in the
GUI.
Chests require requests + transaction bug-fixes.
Tweaked physics for head-on, equal velocity collisions.

Head-on, equal velocity collisions would previously cause agents to
stick together. Added minimum for velocity in collision resolution.
Memory/inventory trades implemented.

Demonstrated in pucks.worlds.dev.world8,
Added docstrings and cleaned up physics.clj.
Documentation, simpler promises, ai.world6.

Added pucks.worlds.dev.world6.
Began documenting transfers in readme.
Simplified promises, which will always be for adding things to memory.
More documentation, version bump.