Merge upstream PRs

[Miradorn]

There are MRs upstream that solve existing problems:

• Don't create compile-time dependencies to embedded schemas #86
  • done in Expand types aliases to avoid compile-time deps #8
• Traverse nested PolymorphicEmbed errors #78
  • done in Add traverse_errors fixes from upstream #12
• nil on data when calls from ex-machina #76
• WIP: ex_machina support #26

Also look at these issues:

• Polymorphic embeds are not kept as changesets but are turned into structs - as opposed to what happens with ordinary embeds · Issue #74 · mathieuprog/polymorphic_embed

[tmjoen]

There has been a lot of great work in this fork as well:

https://github.com/steffende/polymorphic_embed