Move efficient concat to stdlib

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/action.ml

@@ -280,11 +280,6 @@ let prog_and_args_of_values p ~dir =
 
 module SW = String_with_vars
 
-let concat = function
-  | [] -> ""
-  | [x] -> x
-  | xs -> String.concat ~sep:"" xs
-
 module Unexpanded = struct
   module type Uast = Action_intf.Ast
     with type program = String_with_vars.t
@@ -693,7 +688,7 @@ let rec exec t ~ectx ~dir ~env ~stdout_to ~stderr_to =
     exec t ~ectx ~dir ~stdout_to ~stderr_to
       ~env:(Env.add env ~var ~value)
   | Redirect (Stdout, fn, Echo s) ->
-    Io.write_file fn (concat s);
+    Io.write_file fn (String.concat s ~sep:"");
     Fiber.return ()
   | Redirect (outputs, fn, Run (Ok prog, args)) ->
     let out = Process.File fn in
@@ -710,7 +705,7 @@ let rec exec t ~ectx ~dir ~env ~stdout_to ~stderr_to =
     redirect ~ectx ~dir outputs Config.dev_null t ~env ~stdout_to ~stderr_to
   | Progn l ->
     exec_list l ~ectx ~dir ~env ~stdout_to ~stderr_to
-  | Echo strs -> exec_echo stdout_to (concat strs)
+  | Echo strs -> exec_echo stdout_to (String.concat strs ~sep:"")
   | Cat fn ->
     Io.with_file_in fn ~f:(fun ic ->
       let oc =

src/stdune/string.ml

@@ -214,3 +214,8 @@ let enumerate_gen s =
 
 let enumerate_and = enumerate_gen "and"
 let enumerate_or  = enumerate_gen "or"
+
+let concat ~sep = function
+  | [] -> ""
+  | [x] -> x
+  | xs -> concat ~sep xs