Potential refactor: use EnumMap for Counters

So I was going to add a constructor `new Counters(Counter counter)` to reduce verbosity on a handful of methods that have to use a `Counters` argument just to add a single counter, and realized that there's a lot more potential improvement here.

Although we use the enum `CounterType` for creating instances of counters, the actual objects are handled in most places by their names which are Strings.

`Counters` extends `HashMap` mapping counter names as strings to Counter objects which have a `String name` and an `int count`. I'm not sure why it's set up this way but it doesn't seem very efficient to me.

I think it could be adjusted so that `Counters` is instead an `EnumMap`, without too much trouble. Any thoughts?

(Probably much harder to adjust the `Counter` object and no clear payoff for doing so.)

Start by tracing the Counters, Counter, and CounterType classes and their usages throughout the project. Determine whether replacing string-keyed handling with CounterType-keyed handling is viable, then define the affected API and migration scope before making changes; completion requires agreement on the design and a consistent update of all affected uses.

Written by the indexing model from the issue text.