Add a "move" operation

[samoht]

No description provided.

[samoht]

This is needed by most filesystem-like protocol to ensure atomicity.

[samoht]

It is now easy to define move with 1.0:

let move t ~info src dst = 
  S.with_tree t ~info [] (fun tree ->
    S.Tree.find_tree tree src >>= fun v ->
    S.Tree.remove tree src >>= fun tree ->
    S.Tree.add_tree tree dst v
  )

Byt default with_tree will use the Test_and_set strategy, e.g. this will ensure that the move is done atomically. It is possible to define a more fined-grained scope for the transaction by using the longest common prefix between src and dst instead of [] to avoid live-locks.