Sandstone: Fast incremental Haskell builds using dynamic derivations

So far, efforts to use Nix as a build system have suffered from either insufficient incrementality or excessive evaluation time.

Sandstone is a new tool to plan Haskell builds using Nix that achieves full incrementality without affecting eval times at all. It does this by using the new experimental "dynamic derivations" to create fine-grained build steps without involving the Nix language *at all*. 

We hope that Sandstone will kick of a new era of language-specific tooling using Nix!