Implementing clone

[XiangpengHao]

Congee can't be cloned (at least not with https://doc.rust-lang.org/std/clone/trait.Clone.html) due to the linearizability requirement (#8 (comment)).

Consider two threads:

T1:  ------|<----------- clone ---- ------>|-----
T2: ----------|insert(0)|---|insert(1000)|------

The cloned tree from T1 may contain 1000 but not 0, violating linearizability.

Two ways to mitigate this:
(1) Provide a fn clone(&mut self), and ask the caller to ensure no concurrent access. Since we don't provide a safe way to get mutable reference, users must go into unsafe (even UB) to get the mutable reference.
(2) Users can do a range scan over the tree and reconstruct the tree using the records scanned. Each range scan will read several records in a linearizable manner; to scan the entire tree, users must call range scan multiple times. There's no guarantee on multiple scans to be linearizable, but this is a performance-correctness tradeoff.

(I personally don't believe anybody needs linearizability over the entire tree clone; if such thing happens, I don't believe there's any better method than locking the entire tree)
So cloning over congee is a bad idea and if you frequently uses clone, congee might not be best data structure.