Support generic custom serialization

[s-ludwig]

Currently there are three ways to customize how a type gets serialized: toString/fromString, toJson/fromJson and toBson/fromBson

It would be nice to have a way to customize the result without using a specialized output format. An idea could be to support arbitrary T serialize() and deserialize(T) methods, where T is the representation type that gets serialized.

See also #617

[UplinkCoder]

We already have vibed.data.serialization.
It also implicitly defines a serialization interface.

[s-ludwig]

Implemented by 7d22466. Possibly there is still a better name than toSerializedValue.

@UplinkCoder, I think I just didn't express myself well. This not about serialization per se, but about defining a different representation of a value, that forms the basis for serialization.

[s-ludwig]

Maybe a better name: toRepresentation