Separate libraries

[tfausak]

I don't really have any experience working with multiple libraries in a single package. I figured I could use this package as a test bed for that. This PR breaks up the previously monolithic package into separate libraries:

• unstable: This is where everything actually lives. Previously it was just a different module namespace (Saturn.Unstable.*). Now it's also a separate package component (saturn:lib:unstable).
• saturn: This is the default public library and now it only exports the Saturn module. Everything else lives in unstable.
• spec: This is where all the tests live. Since there are useful functions (like arbitrary, shrink, and new) defined for each type, it seemed like a good idea to have this live in a library rather than a test-suite.

In terms of behavior, everything should be the same as before. I only changed package organization and tests.

[tfausak]

One (major) downside of this approach is that Cabal's solver isn't really aware of sub-libraries: haskell/cabal#4087

That means the dependency solver tries to find a solution for all sub-libraries, even if you don't actually depend on them. Practically, that means you can only use saturn if your versions of hspec and QuickCheck match up, even if you don't depend on saturn:lib:spec.