Make Encode return a result

[FSMaxB]

fixes #2730

~~Still working on this, but wanted to make a draft PR where I can push my current progress.~~

My approach was to incrementally turn return values into Result in one layer, but keeping the panic by calling .expect(...) on the layers that use those, then go to the next layer and turn the .expect(...) into ? and push the panics up further and further until they're gone.

I'll leave some open questions as review comments.

[FSMaxB]

Getting the macros to work again is really tricky as it turns out!

[FSMaxB]

I believe I have addressed all review comments, so this is ready for another round of review.