Add Nix wrappers

[4e6]

Nix

close #3
close #4
close #5
close #14
close #16

Nix infrastructure.

stackage2nix

Directory contains ready-to-use stackage2nix wrapper with
--all-cabal-hashes and --lts-haskell flags set up. It also has all
required executables on PATH.

Install stackage2nix:

$ nix-env -i -f ./stackage2nix

$ stackage2nix
Missing: (--resolver ARG | STACK_YAML)

stackage

Directory contains derivation that creates Stackage LTS from lts argument.

Build lts-9.0:

$ nix-build --argstr lts lts-9.0 ./stackage

$ ls result
configuration-packages.nix  packages.nix

haskellPackages

Directory contains Stackage LTS packages generated by stackage2nix from this
project's stack.yaml.

lib.nix

Provides Nix expressions for hackage-db, all-cabal-hashes and lts-haskell
sources.

[4e6]

PR addresses usability issues. With Nix wrapper user is not required to set mandatory --all-cabal-hashes and --lts-haskell flags. Wrapper delegates maintenance of these service repositories to Nix.
cc @nc6