Cache rlp in deserialized Serializable

[ethereum]

Cache rlp in deserialized Serializable

[ulope]

Raising NotImplementedError in a setter is actually not needed. Simply not defining a setter will suffice (and cause an AttributeError in case setting the property is attempted).

[jnnk]

• Serializable now has attributes rlp_ and mutable_ (trailing underscore to distinguish from possibly clashing field names)
• if mutable_ is False, attempts to set field values throw a ValueError. Note that (1) mutable_ is not recursive (i.e. block.mutable_ and block.header.mutable_ are independent, although they have the same value after initialization) and (2) changes of existing objects are not tracked (e.g. it does not prevent appending to a list).
• rlp.decode(s) sets rlp_ to s and (implicitly by deserialize) mutable_ to False.
• rlp.encode(obj) returns obj.rlp_ if it's there
• rlp.encode(obj, sedes=SomeClass) ignores rlp_ (because different serializers can lead to different rlp, e.g. rlp.encode(tx, Transaction) vs rlp.encode(tx, UnsignedTransaction))

To mutate an immutable object, set mutable_ to True and (if the changes are persistent) rlp_ to None.