stdenv -> module system

[roberth]

Move away from stdenv and its hooks.
Bash provides mediocre means of composition at best. I've helped someone troubleshoot an error that got its position distorted by eval. It's left us severely disillusioned.
Another potential issue is environment variables leaking from the effect's stdenv into nix-shell/nix develop.

Instead, effects could be composed using the module system allowing the reuse of some NixOS idioms, with the benefits of more type checking and a cleaner process environment.

• make it work
• make all effects use modules
• clean up the public interface a bit

[roberth]

Still keeping some of stdenv, which is useful, but can now start porting things to module style, since #107