feature: sparse/lightweight internal representation of agents

an old 2011 writeup of mine:

An internal technical improvement that will permit the addition of a variety of user-visible improvements is changing the engine's internal representation of agents, including patches in the patch grid, to use a sparser, more lightweight representation so that adding agent variables and using large numbers of agents will become more practical.

We propose to develop new underlying data structures such that:

• Worlds with large numbers of patches will be cheap to create and will use memory proportional only to the amount of informationactually stored in the patches, rather than directly proportional to the patch count.
• Unbounded patch grids, in which turtles can wander an arbitrary distance from the origin without ever encountering a world boundary, will become possible for the first time.
• New properties can be added to turtles and patches without increasing memory usage for models in which properties are not used. For example, users often ask for control over the font size of labels attached to turtles, but so far we haven't added that because it would decrease speed and increase memory usage for all models.

No files, tests, or entry points are named. Start by reading the issue's listed goals for sparse patches, unbounded grids, and optional agent properties, then locate the current agent and patch representation; done requires a concrete design and validation for the stated memory and capability goals.