Restate the problem as matrices

[daffidwilde]

There are a couple issues that would be nice to address:

• time complexity of the algorithms hospital_resident performance #26
• scalability RecursionError for deepcopy with 400x400 matching task #139

I'm thinking that the package needs a rework. I lent very heavily into making it a feasible teaching tool and ended up with some very difficult innards...

Is there a way to restate the problems (and algorithms) as matrices? My instinct is yes.

If there is, we can use vectorised operations to speed things up (everything becomes index comparisons, I think). We could also expand these matrix-based solutions to use something like dask for enormous, out-of-memory instances...