Add a PursuitEntry data type

Add a data type isomorphic to the JSON data produced. This makes it much
easier to work with the data in ghci, for instance.

The motivation for this was that I wanted to get a list of all modules that are in the current pursuit data set, and looking at what `Data.Aeson` exports, it seems they expect you to define your own type with a `FromJSON` instance, rather than to work with `Data.Aeson.Value` values directly. I expect inspecting, querying, and modifying pursuit data in the repl is something I'll need to do more of in the future, and so I think this change is worth making.