Implements the Access behaviour for a struct.

Structs do not support the Access callbacks on their own, so get_in/2, put_in/2 and update_in/2 cannot reach into them with the bracket and dot syntax. Adding use Ark.StructAccess delegates the Access callbacks to Map, so the struct works with those macros.

defmodule MyApp.Settings do
  use Ark.StructAccess
  defstruct [:theme, :locale]
end

settings = %MyApp.Settings{theme: :light}
put_in(settings.theme, :dark)
# => %MyApp.Settings{theme: :dark, locale: nil}

The pop callback raises, since removing a key would leave a value that is no longer the struct.