Use multimethods for custom hydration functions

[camsaul]

Instead of tagging them with metadata it would be better if we could make them multimethods that dispatch off of the model as well as the key they are hydrating. That way different methods could be used to hydrate the same key for instances of different models. The approach also seems a little cleaner than having hydrate look through all namespaces for functions with matching metadata

There are two things that might make this tricky:

• The difference between single hydration and batched hydration functions would have to be addressed somehow (perhaps with two multimethods?)
• This change would require reworking things a bit for people using the library

[camsaul]

I'm actually of the opinion that everything (including things like pre-insert!) should be implemented with multimethods instead of protocols. That would be a big breaking change, however.

[tlrobinson]

Is there a workaround for "different methods could be used to hydrate the same key for instances of different models"?