irmin-lwt: include Irmin.Branch.S in S.Branch Mirror the Irmin 3.11 [S.Branch] signature, which terminates with [include Branch.S with type t = branch] to expose [val main], [val is_valid], and [val t : t Irmin.Type.t] alongside the I/O functions. Without this, [Store.Branch.main] is unreachable through [Irmin_lwt.S.Branch].