fn eq(&self, other: &Self) -> bool {
        self.cmp(&other) == Equal
    }
Looks up a given key in a (sub)tree headed by the node, recursively.
Returns a `Found` with the handle of the matching KV, if any. Otherwise,
returns a `GoDown` with the handle of the leaf edge where the key belongs.

The result is meaningful only if the tree is ordered by key, like the tree
benchmarks/haystacks/code/rust-library.rs