Use cabal to work with Haskell sources

1. Build everything

   cabal build all

2. Build library only

   cabal build ClickHaskell

3. Run tests

   cabal run test

   Note that: test requires database

Use nix to run routine actions

1. Initialize database

   to work with database dependent code

   nix run

   (delete ./data dir to reinitialize database)

2. Initialize database and run tests

   nix run .#testing

   (delete ./data dir to reinitialize database)