Alpha: add rpc to get seed of a cycle of a block

src/proto_alpha/lib_protocol/src/alpha_context.mli

@@ -473,14 +473,21 @@ end
 
 module Seed : sig
 
+  type seed
+
   type error +=
     | Unknown of { oldest : Cycle.t ;
                    cycle : Cycle.t ;
                    latest : Cycle.t }
 
+  val for_cycle:
+    context -> Cycle.t -> seed tzresult Lwt.t
+
   val cycle_end:
     context -> Cycle.t -> (context * Nonce.unrevealed list) tzresult Lwt.t
 
+  val seed_encoding : seed Data_encoding.t
+
 end
 
 module Contract : sig

src/proto_alpha/lib_protocol/src/alpha_services.ml

@@ -11,6 +11,35 @@ open Alpha_context
 
 let custom_root = RPC_path.open_root
 
+module Seed = struct
+
+  module S = struct
+
+    open Data_encoding
+
+    let seed =
+      RPC_service.post_service
+        ~description: "Seed of the cycle to which the block belongs."
+        ~query: RPC_query.empty
+        ~input: empty
+        ~output: Seed.seed_encoding
+        RPC_path.(custom_root / "context" / "seed")
+
+  end
+
+  let () =
+    let open Services_registration in
+    register0 S.seed begin fun ctxt () () ->
+      let l = Level.current ctxt in
+      Seed.for_cycle ctxt l.cycle
+    end
+
+
+  let get ctxt block =
+    RPC_context.make_call0 S.seed ctxt block () ()
+
+end
+
 module Nonce = struct
 
   type info =

src/proto_alpha/lib_protocol/src/alpha_services.mli

@@ -9,6 +9,12 @@
 
 open Alpha_context
 
+module Seed : sig
+
+  val get: 'a #RPC_context.simple -> 'a -> Seed.seed shell_tzresult Lwt.t
+
+end
+
 module Nonce : sig
 
   type info =