Use fixed-width integers to represent days

Currently https://hackage.haskell.org/package/time-1.8.0.1/docs/Data-Time-Calendar.html#t:Day is represented in memory as an `Integer`, and many similar functions like `fromGregorian` which interface with `Integer`s. While this is admirably future-proof (we can deal with years larger than the year 2,147,481,790 AD!) I think we will be using intuitionistic type theory sooner than we will need those dates :)

Using something like `Int32` would give us nearly the same flexibility, better `Storable` instances, interfacing with C and serialization/deserialization. I'm not sure how much this will break downstream code, but I have a feeling most people are using `fromIntegral` to interface with the API anyways, and this change will actually lessen the amount of marshalling people need to do.