Common serde functionality

[tarcieri]

Context: RustCrypto/nacl-compat#27

Several of our crates have serde integrations, and it'd be nice to expand that.

One thing that'd be really nice to have is some common functionality for for writing serializers/deserializers which don't depend on serde_derive, particularly if this functionality could make writing serializers/deserializers declarative with #[serde(try_from = "...", into = "...")].

Some of the hard lessons were learned writing the serializers for the ed25519 crate which presently depends on the serde_bytes crate for serialization. That was largely a workaround for the inadequacies of the original tuple-based serializers, which resulted in bloated documents when generating CBOR.

Perhaps this could go in crypto-common? Some common concerns:

• Lack of const generics support in serde and difficulty serializing arrays larger than 32 elements. serde_bytes addresses this somewhat, but it'd be nice not to have to loop in an additional dependency, and I think a simpler solution is possible.
• Choosing what formats to optimize for: serializing a byte array as a tuple of N u8 elements makes for a compact bincode representation, but an extremely bloated CBOR serialization for example. We should have a single place to test commonly used binary serializers, and choose which ones we want to optimize for. I would suggest optimizing for CBOR at the cost of bincode.
• Support for choosing different encodings for binary vs human readable formats: in several places, the elliptic-curve crate uses a hex serialization for "human readable" text-based formats like JSON/TOML, and binary otherwise. While I think this sort of autodetection should be opt-in, it'd be nice to have a single place where it's implemented.