IPLD Block Abstraction

[Stebalien]

Currently, the actor's runtime passes around RawBytes which are assumed to be CBOR (or "nothing" if empty). However:

1. We'd like to better support IPLD_RAW parameters (e.g., in the EVM). I.e., not DAG_CBOR.
2. We should explicitly support a more "typed" nothing. Currently, we use an "empty" RawBytes for this, but that's not actually valid CBOR.

Proposal: Introduce a new IpldBlock type and change the runtime/actor methods to use Option<IpldBlock> in parameters and return values (instead of RawBytes).

pub struct IpldBlock {
    pub codec: u64,
    pub data: Vec<u8>,
}

impl IpldBlock {
    pub fn deserialize<'de, T>(&'de self) -> Result<T, ActorError>
    where
        T: Deserialize<'de>,
    {
        match self.codec {
            IPLD_RAW => todo!("deserialize as 'bytes' only"),
            DAG_CBOR => todo!("deserialize into arbitrary types as cbor"),
            _ => Err(actor_error!(serialization; "unsupported codec {}", self.codec)),
        }
    }
    pub fn serialize<T: Serialize>(codec: u64, value: &T) -> Result<Self, ActorError> {
        // Attempt to encode T with the specified codec.
        // ...
    }
    pub fn serialize_cbor<T: Serialize>(codec: u64, value: &T) -> Result<Self, ActorError> {
        self.serialize(DAG_CBOR, value)
    }
}

To avoid having to make too many manual changes up front, I'm proposing the following "tooling assisted" upgrade path:

1. Mass rename Runtime::send to Runtime::send_cbor.
2. Mass rename ActorCode::invoke_method to ActorCode::invoke_method_cbor.
3. Introduce a new Runtime::send, making Runtime::send_cbor a special case that only handles cbor.
4. Introduce a new ActorCode::invoke_method call that delegates to ActorCode::invoke_method_cbor by default.

This will create a fairly large diff, but shouldn't require too much manual work.