Organize Agents

[eriq]

The organization of agents is a mess.
They are implemented in a bunch of different files without much structure.
Often non-agent code will be in an agent file.

Sort all this out.

[eriq]

One agent to a file, all in the agent package.
Note that this issue is not about loading agents.

[eriq]

I'll take this one.

[eriq]

Complete as of 790e7f1.
We still may move around some student code some as we update the autograder and assignments, but the non-student code it pretty organized.