Make the state argument explicit

This is just a thought. With this change, the `state` argument must be declared by the user if they want to use it inside `defcall`'s body:

```elixir
    # OK
    defcall api_with_params_and_state(num, state) do
      reply(state + num)
    end

    # Not OK
    defcall api_with_params_and_state(num) do
      reply(state + num)   # <-- line 12
    end
    # ** (CompileError) test/genserver_test.exs:12: function state/0 undefined