Exits component redesign

[dmlary]

Previously Exits contained references to "passage" entities that had a destination, and optionally a closable component. This caused a couple of issues:

• To connect two room entities, you needed two passage entities to create a bi-directional link (which is most common)
  • This causes the number of entities in the system to balloon when we pull in a map of ~10k rooms (ended up with 30k entities)
• When you opened a door in one room, the door on the other side would still be closed because the closed state resided in each independent passage entity

So to fix both of these issues, I tossed out the idea of a passage.

For each direction from a room, the Exits component has a destination in that direction (entity id), and optionally a door entity that maintains the closable component for both sides of the door.

- id: example:room/1
  base: morrow:room
  components:
  - exits:
      east: example:room/2
      east_door: example:door/1-to-2

- id: example:room/2
  base: morrow:room
  components:
  - exits:
     west: example:room/1
     west_door: example:door/1-to-2

- id: example:door/1-to-2
  base: morrow:door/closed

fixes #12