Remove separate validation array

[mkeeter]

Right now, we have two types with the same shape:

/// Results of validating a single block
#[derive(Copy, Clone, Eq, PartialEq, Debug)]
pub(crate) enum Validation {
    /// The block has no hash / context and is empty
    Empty,
    /// For an unencrypted block, the result is the hash
    Unencrypted(u64),
    /// For an encrypted block, the result is the tag + nonce
    Encrypted(crucible_protocol::EncryptionContext),
}

#[derive(Debug, PartialEq, Copy, Clone, Serialize, Deserialize)]
pub enum ReadBlockContext {
    Empty,
    Encrypted { ctx: EncryptionContext },
    Unencrypted { hash: u64 },
}

The DownstairsIO stores both of these types:

struct DownstairsIO {
    // other members elided
    data: Option<RawReadResponse>, // contains blocks: Vec<ReadBlockContext>
    pub hashes: Vec<Validation>,
}

By the time we write to data, the block contexts have already been validated, so storing them again in hashes adds unnecessary duplication (and they're guaranteed to be the same).

This PR removes the duplicate Validation hashes, using the RawReadResponse::blocks member instead.

There are small changes to GuestBlockRes::transfer_and_notify and Buffer::write_read_response, because we can't take the entire Option<RawReadResponse>; instead, we pass the &mut BytesMut separately, so it can be sent back to the host.