Update raw.hpp so that we can serialize and deserialize const types

[greg7mdp]

Some types are only set in the constructor and never modified. So they are logically const and could be accessed from multiple threads safely.

Using this would allow us to remove some friend declarations (we use private as a way to prevent unchecked access to non thread-safe members).

Ideally we would make the member type const.
The serialization is what trips us up. I feel that the unpack functions in raw.hpp should accept to unpack into const members of structs (logically deserializing to recreate a value which is never modified after creation). Right now we cannot mark non-funadamental types that needs to be serialized const.