Write formal definitions of schedule datatypes

[triangle-man]

Supercedes #10 .

Whatwhat's model of the world is captured by the Schedule.schedule type, which consists of people, projects, etc. What is the correct domain model represented by those types?

[triangle-man]

Going offtopic already: should the type Schedule.schedule in fact be called Schedule.t, following OCaml convention? Maybe not: the Schedule module doesn't just define a type and associated operations: what it's really about is creating a schedule object.

[mhauru]

Maybe not: the Schedule module doesn't just define a type and associated operations: what it's really about is creating a schedule object.

Not very familiar with OCaml conventions, but I would lean towards this point of view.

[triangle-man]

While pursuing this issue, I got caught up in more general questions of where to define the types. See #44

[triangle-man]

See docs in domain.mli. Suspect we may have to revisit.