Clean up our core-macro implementation.

[jkelleyrtp]

Feature Request

The core-macro crate is rather gnarly now and is a little too complex to wade through.

We should:

• remove the deserializer trait in favor of just functions
• collapse as much codegen as possible - especially around the owned builder stuff since that's leading to some code duplication
• refactor out the impl so it's not such a huge file
• be very explicit with functions and document them

Codegen is a particularly sensitive part of the codebase so we should 1) be as clear as possible and 2) test it.