Port PureSwift to spago and PureScript 0.13

[jbmusso]

Just found out about this branch too, which might be of interested to others.

Current status:

• nix-shell fails due to spago and spago2nix being imported locally, outside of the project (see https://github.com/paulyoung/pureswift/commit/6c3bc6a51757c60e7c49d5ad308d2d0f0a18431e#diff-f1e7ac253697b80970036bc233593f02).

We could be tweaking something from https://github.com/justinwoo/spago2nix. I have some experience with nix, I'd be happy to help if needed.