Alpha/Refactor: use return_*

Replaces some allocations of common return values by preallocated constants.
This is a cosmetic change that does not change the semantics.

src/proto_alpha/lib_protocol/src/amendment.ml

@@ -212,7 +212,7 @@ let rec longer_than l n =
 let record_proposals ctxt delegate proposals =
   begin match proposals with
     | [] -> fail Empty_proposal
-    | _ :: _ -> return ()
+    | _ :: _ -> return_unit
   end >>=? fun () ->
   Vote.get_current_period_kind ctxt >>=? function
   | Proposal ->

src/proto_alpha/lib_protocol/src/roll_storage.ml

@@ -297,7 +297,7 @@ module Delegate = struct
       | None ->
           (* This case is only when called from `set_active`, when creating
              a contract. *)
-          return false
+          return_false
 
   let add_amount c delegate amount =
     ensure_inited c delegate >>=? fun c ->

src/proto_alpha/lib_protocol/src/script_ir_translator.ml

@@ -1617,7 +1617,7 @@ and parse_instr
     let log_stack ctxt loc stack_ty aft =
       match type_logger, script_instr with
       | None, _
-      | Some _, (Seq (-1, _) | Int _ | String _ | Bytes _) -> return ()
+      | Some _, (Seq (-1, _) | Int _ | String _ | Bytes _) -> return_unit
       | Some log, (Prim _ | Seq _) ->
           (* Unparsing for logging done in an unlimited context as this
              is used only by the client and not the protocol *)
@@ -1625,7 +1625,7 @@ and parse_instr
           unparse_stack ctxt stack_ty >>=? fun (stack_ty, _) ->
           unparse_stack ctxt aft >>=? fun (aft, _) ->
           log loc stack_ty aft;
-          return ()
+          return_unit
     in
     let return :
       context -> bef judgement -> (bef judgement * context) tzresult Lwt.t = fun ctxt judgement ->

src/proto_alpha/lib_protocol/src/voting_services.ml

@@ -111,8 +111,8 @@ let register () =
   register0 S.current_proposal begin fun ctxt () () ->
     (* this would be better implemented using get_option in get_current_proposal *)
     Vote.get_current_proposal ctxt >>= function
-    | Ok p -> return (Some p)
-    | Error [Raw_context.Storage_error (Missing_key _)] -> return None
+    | Ok p -> return_some p
+    | Error [Raw_context.Storage_error (Missing_key _)] -> return_none
     | (Error _ as e) -> Lwt.return e
   end