Alternative project specification via stack.yaml + freeze file

[kirelagin]

Parse project specification from stack.yaml. Do this the easy way by just reading it as yaml (kinda the way stackage2nix does it; it would be better to use stack the library of course, but it will take too much time).

The missing parts (hashes of git dependencies) will come from a separate file in the root of the project, hashes.yaml.