Add mutable record

[mildred]

Allow identifying mutable objects given the hash of an immutable
record definition. To avoid using too much codes, the record definition could be defined to require a multicodec header.

[daviddias]

As I brought up in #37 (comment)

IPRS is just data, not a format, it is not required to create a format for something that doesn't have a different serialization format nor has a specific way to resolve through. I understand that the documentation could be more clear on this.