This change adds a `materialized` argument to the functions that
generate nix files with `plan-to-nix` and `stack-to-nix`.
* Provides `builtins.trace` messages with pointers on how to do so.
* Includes a `checkMaterialized` option that can be used
to verify the materialized files.
* Documents how to script updates with `plan-nix` or `stack-nix`.
* Outputs warnings if materialized files are used without hash
(or without an `index-state` for cabal projects).
* Provides materialized nix files for `alex`, `happy` and `hscolour`
