Checklist for implementing new Serializers

There's a lot to think about to make sure a new Serializer has all the features, so here's a checklist:

• Implement core functionality (packing/unpacking logic).
• Implement prepack/preunpack when needed. For container type Serializers, consider something similar to what CompoundSerializer does to optimize only if any of the sub-serializers need a special prepack/preunpack call.
• Check if you need to handle TypeVars at all.
• Implement any transformations needed for type_checking.annotated to automatically unwrap the Serializer correctly. See StructuredSerializer for an example - hinted Structured-derived classes get automatically wrapped with a StructuredSerializer.
• Tests, try to get 100% coverage to make sure you've covered all the edge cases.
• Documentation.