Add `BatchId` instances and hide constructor

[z0isch]

BatchId does not currently have some of the usual suspects for instances (Show, Eq, Ord ...) which makes it tough to integrate into other date types.

It also exposes the constructor which is probably not the way we want a client to interact with this data type.

[pbrisbin]

It seems at some point Show and Eq were added, though not Ord. I'm also not particularly bothered by the constructor being exposed. So I'm going to close this issue, but please re-open if you disagree @z0isch.