Added default implementations for 'Serializer' and 'Deserializer' trait by raising error if not overridden

[AlexSherbinin]

No description provided.

[oli-obk]

Can you talk a bit about your motivation for this change? Ideally with some links to examples that are hard, annoying or footgunny without this?

[AlexSherbinin]

1. Default method implementations already exist for the u128 and i128 types.
2. There is a significant boilerplate involved with using the Impossible serializer in SerializeTuple, SerializeSeq, and similar cases.

I created this PR while transitioning this crypto library from an internal trait for hash function parameter serialization to Serde, and encountered the aforementioned issue with boilerplate.

[oli-obk]

1. Default method implementations already exist for the u128 and i128 types.

That's only due to them being introduced later, so we can't add them without a breaking change or a default impl

2. There is a significant boilerplate involved with using the Impossible serializer in SerializeTuple, SerializeSeq, and similar cases.

These are rather special cases. Most serializers will want to implement all of them, and with default impls it's annoying to make sure you got them all

[AlexSherbinin]

These are rather special cases. Most serializers will want to implement all of them, and with default impls it's annoying to make sure you got them all

Rust analyzer has code action to implement all trait members, even with default impl.