Add opaque typespecs

[schutm]

I've added the possibility to have @type-specs added as @opaque-specs. I like to keep my structures hidden, so I'm able to change the types and other stuff.

So the following will create a opaque typespec for Person.t().

defmodule Entities do
  use Strukt

  defstruct Person do
    @opaque_fields true

    field :name, :string, required: true

    def name(person), do: person.name
  end
end

I'm not sure, whether:

• this is a good addition to this library;
• if the chosen interface is desirable;
• the implementation is any good.

Any thoughts are appreciated.