Introduce hash addressable content interface

[loongy]

It is common, when using this library, that someone wants to use a hash to uniquely identify an object (for example, a block in a blockchain). However, this is not always the same as the hash of the bytes of the marshaled object (blocks in blockchains contain signatures, but you cannot include these when hashing a block so that you can sign it, because this is a recursive definition with no terminal case).

This library should define an interface that can be implemented by any type that knows how to convert itself to a hash. By convention, this hash is used to uniquely identify the object, and used to sign the object (prove authenticity). It does not have to be the same as the hash of the marshaled object.

type Content interface {
    Hash() Hash
}

[loongy]

We ended up with

type Content interface {
    surge.Marshaler
    Hash() (Hash, error)
}

and the supporting helper type

type Blob struct { .. }

for types where the hash is known to be a hash of the binary representation (this is often the case, but should not be assumed to be the case).