Ergonomics request: encapsulate the Cache in CacheDB<DB> to allow it to be used independently

[prestwich]

It's often useful during simulation to separate cached updates from the underlying DB, so that they can (e.g.) be absorbed into a higher-level cache.

Requested refactor would look something like this:

pub struct Cache {
    pub accounts: HashMap<Address, DbAccount>,
    pub contracts: HashMap<B256, Bytecode>,
    pub logs: Vec<Log>,
    pub block_hashes: HashMap<U256, B256>,
}

pub struct CacheDB<ExtDB> {
    cache: Cache,
    db: ExtDb
}

[rakita]

This makes sense

[prestwich]

I can prep a PR if it wouldn't interfere with any ongoing refactor work