* Add caltrop component for spikey floor objects
The caltrop component now can be added to any crossable atom, and it'll
act like a shard of glass, or a d4. Additional flags are possible for it
to bypass shoes or ignore people who are walking.
This means d4 don't reimplement shard logic, and also open the window
for caltrop grenades later.
Also, it taught me how components work.
* Code review I
* Caltrop damage is 4
* Cactus hurts
* Whoops
* Ignore restraints = true
