multi: Move each serializer into its own package

[matheusd]

This PR moves each serializer (or set of related serializers) into its own internal package.

This helps organize the code and makes it more obvious what is needed to make each serializer work.

Some serializers (such as the shamaton ones) hook directly into the structure, therefore a copy of it (instead of a direct reference) needs to be used.

The standard library serializers are moved to the stdlib package.