System Lookup Interface

[pathunstrom]

Right now, when one system needed another system, there's no good way to do it.

As part of this issue, we should build an interface to allow this.

Considerations:

• Should be non-blocking: A system that doesn't have another system shouldn't lock up and should do its best to keep functioning without the other system.
• Should probably use the existing signalling system to request and respond.
• Consider a simple callback model.