enh/115-efficient-arena-map

[jharwell]

For VERY large arena maps (e.g., 2,000 x 2,000), COSM can request over 48GB of
memory just for the array of cells forming the base of the arena map. Depending
on configuration, most of the arena can be empty (does not contain a block,
cache, nest, etc., on very many cells). So, this is inefficient, and can limit
the size of the swarms that I can use.

A better approach would be to switch to a different underlying data structure,
such as a std::map/std::unordered_map, which I could use to create "sparse"
arrays indexed by an rmath::vector2z object, which will be more computationally
efficient.

This will require some additional checking logic when dropping/picking up a
block, dropping in a cache, etc., because the cells will need to be created if
they don't exist. I don't know if they should be destroyed when they don't get
used for N timesteps--maybe ?