Refactor and test in preparation for 1.0 #350
Description
As discussed in RFD 614, we want to make some behavioral changes to the SDK before releasing version 1.0. In advance of those changes, I'd like to refactor the code generation logic a bit, so that behavioral changes are easier to manage. The current code generation logic works well but can be hard to follow, and is sparsely tested at times. Where possible, I'd like to move from long functions that handle many cases to structs that represent small parts of the system, and use methods on each struct to render itself to Go code. We can borrow inspiration from similar libraries like oapi-codegen here.