Provides macro to support Bitfield datatype with Ecto

defmodule User do
  use Ecto.Schema

  import EctoBitfield

  # takes in a list or a keyword list for explicitly setting the mappings
  defbitfield Policies, [:create_user, :update_user, :delete_user]

  schema "users" do
    field :policies, User.Policies
  end
end

Reading:

query = from u in User, where: u.policies == ^[:create_user, :update_user]
#> %Ecto.Queryable{...}

user = Repo.one(query)
#> %User{..., policies: [:create_user, :update_user]}

Writing:

changeset = Ecto.Changeset.cast(user, %{policies: [:create_user]}, [:policies])
#> %Ecto.Changeset{..., changes: %{policices: 1}}

Repo.update(changeset)
#> {:ok, %User{..., policices: [:create_user]}}