Add Block struct to Miden objects

[bobbinth]

Currently, we have a Block struct defined in miden-node. I think this is a foundational enough struct that it should be defined in miden-base. This struct could then be used in various places (e.g., for storing block data on disk, or returning block data via node endpoints etc.).

The block struct should into the block module. It could look something like this:

pub struct Block {
    header: BlockHeader,
    updated_accounts: Vec<AccountUpdate>,
    created_notes: Vec<(BlockNoteIndex, OutputNote)>,
    created_nullifiers: Vec<Nullifier>,
    // TODO: zk proof
}

pub struct AccountUpdate {
    account_id: AccountId,
    new_state_hash: Digest,
    delta: Option<AccountDelta>,
}

pub struct BlockNoteIndex {
    batch_index: u8,
    note_index: u16,
}

A couple of open questions:

1. Should we put BlockNoteTree into the block as well (e.g., as a lazily constructed filed)?
2. The above assumes that #524 is implemented and we can use just AccountDelta for the update details.

[bobbinth]

Closed by #621.