Make default.nix an overlay

Similar to [mozilla-nixpkgs](https://github.com/mozilla/nixpkgs-mozilla).

Benefits:
* familiarity ([this](https://nixos.wiki/wiki/Overlays#Applying_overlays_manually) is all you'd have to do), and then you could just add `dapp` or `seth` to `home.packages` or `environment.systemPackages`.
* makes more sense

Cons:
* Current users might have to change how they import the pkgset
* Do we break someone's workflow?

I think for this to actually work well, we'd have to stop exporting [this](https://github.com/dapphub/dapptools/blob/5f6812169f99a71d12d21229f410238c4c2af964/overlay.nix#L8), otherwise we're actually going to interfere with haskell dependencies from nixpkgs, which can cause lots of downstream rebuilds.