Add timeline packages

[kokobd]

TODOs

• improve comments and descriptions
• remove dependency on string-interpolate
• test if it works across all supported ghc versions
• add proper version bounds

[JackKelly-Bellroy]

For simple packages like timeline and aws-arn, I think that we are better off using nixpkgs rather than haskell.nix.

[JackKelly-Bellroy]

Many nits, but this is looking good.

[lrworth]

Does this package need to be based on time? It seems like any totally-ordered type would work.

Could we call this continuous-map or something and remove the dependency on time?

[JackKelly-Bellroy]

Does this package need to be based on time? It seems like any totally-ordered type would work.
Could we call this continuous-map or something and remove the dependency on time?

Yeah I think if you conceptualise it as a map of non-overlapping intervals but then say in the docs "one common application is timeline data", you get a much more flexible data structure.

Something to consider: Do we want to require that the set of intervals completely covers the range of the key type? Certainly in the timeline case we might want to require that, but an interval map with a default value represents a constant function, and then you can quite reasonably build on top of that.

[kokobd]

It's easy to replace UTCTime with Ord t, I'll do that. But I don't think we should strive to give it a more abstract name now. The original motivation for creating a timeline package is to represent "timelines", and the interval map is just one representation of that. We may support more representations, like pure functions, in the future.

In the long run, IMO, continuous-map is a standalone package that does its job, and timeline is a package built on top of that, to provide different timeline representations.

[kokobd]

Merging. If we have anything else to change, let's do that in another PR.