Add ExMachina.EctoPolymorphicEmbed

[atdrendel]

This is an attempt to make ExMachina work with polymorphic_embed following the lead of this comment.

[westmark]

I was using a local version of the same, and after updating my packages (unsure which caused it) I had to change

&match?({:parameterized, PolymorphicEmbed, _options}, schema.__schema__(:type, &1))

to

&match?({:parameterized, {PolymorphicEmbed, _options}}, schema.__schema__(:type, &1))

for the casting to work

[teamon]

Need to account for array of polymorphic embeds:

  defp schema_polymorphic_embeds(schema) do
    Enum.filter(schema.__schema__(:fields), fn field ->
      case schema.__schema__(:type, field) do
        {:parameterized, {PolymorphicEmbed, _options}} -> true
        {:array, {:parameterized, {PolymorphicEmbed, _options}}} -> true
        _ -> false
      end
    end)
  end