Use a Tuple in splitAt

[hdgarrood]

Fixes #69.

This introduces a dependency on tuples, but tuples only depends on foldable-traversable itself, so this change shouldn't add any additional transitive dependencies.

[paf31]

Another option is to use a record { before, after }, but either way we'll need to do a major version bump due to the type change.

[hdgarrood]

I guess a record is more consistent with the other core libraries, e.g. uncons from both Data.Array and Data.String. I'll change it.

[hdgarrood]

I guess strings is quite low down in the core hierarchy. Do we want to save this for later?

[paf31]

Theoretically, once we're switched over to psc-package instead of Bower, these sorts of changes could be non-breaking for the package set, even though it is breaking for the package itself. Maybe if we wait, we could try that out there?

[hdgarrood]

I don't see how that could work - what if people are using splitAt with its current type in their applications? I'll send a new PR anyway and we can merge it / release it when it's convenient.

[paf31]

Sorry, you're right. What I should have said is - we'd bump the version on the package set, but we wouldn't necessarily need to rerelease all reverse dependencies. As long as they were compatible, the existing versions could stay in the new package set.