45: Fix struct newtype deserialization (and add tests) r=nox a=samsieber

Fixes #41

I only had to fix the deserializer - the serialization already works. So now they work the same way - you can serialize something and then deserialize it losslessly. 

I also added tests for serialization and deserialization. Let me know if there's anything you'd like changed.

Co-authored-by: Sam Sieber <swsieber@gmail.com>