Refactor lookup logic

[frisitano]

The lookup logic heavily overlaps between both TreeDB and TreeDBMut, specifically lookup(..) and lookup_leaf_node(..) methods. We should extract the logic to a separate module such that the logic can be shared. In the module we should create a struct that implements both these methods. The struct should look something like:

pub struct Lookup<'db, const D: usize, H: Hasher> {
    storage: NodeStorage<H>,
    db: &'db mut dyn HashDB<H, DBValue>,
    root: NodeHash<H>,
    null_nodes: HashMap<H::Out, Node<H>>,
    recorder: Option<core::cell::RefCell<&'db mut dyn TreeRecorder<H>>>,
}

We should then modify both TreeDB and TreeDBMut to leverage the methods exposed from this struct.

It may be useful to implement the From trait on TreeDB and TreeDBMut for Lookup such that we can easily construct a Lookup struct using Lookup::from(..) when doing lookups.