Defines thin contracts with convention-driven call delegation to internal implementations.

Example

iex> defmodule Invoicing.CreateUser do
iex>   def call(name), do: {:ok, name}
iex> end
iex>
iex> defmodule Invoicing do
iex>   use Modular.Delegate
iex>
iex>   defcall create_user(name)
iex> end
iex>
iex> Invoicing.create_user("Mike")
{:ok, "Mike"}