Remove load-time argv parsing

`Conf.action` had side effects as it immediately parsed `Sys.argv` at
load time. This has to be delayed so that we can link the lib in a test
suite for example.

bin/ocamlformat.ml

@@ -68,7 +68,7 @@ let source_from_file = function
   | File f -> In_channel.with_file f ~f:In_channel.input_all
 
 ;;
-match Conf.action with
+match Conf.action () with
 | Inplace inputs ->
     let errors =
       List.filter_map inputs

bin/ocamlformat_reason.ml

@@ -57,7 +57,7 @@ let to_output_file output_file data =
   | Some output_file -> Out_channel.write_all output_file ~data
 
 ;;
-match Conf.action with
+match Conf.action () with
 | Inplace _ -> user_error "Cannot convert Reason code with --inplace" []
 | Check _ -> user_error "Cannot check Reason code with --check" []
 | In_out ({kind; file; name= input_name; conf}, output_file) -> (

lib/Conf.ml

@@ -2172,7 +2172,7 @@ let validate () =
   | Error e -> `Error (false, e)
   | Ok action -> `Ok action
 
-let action = parse info validate
+let action () = parse info validate
 
 let debug = !debug
 

lib/Conf.mli

@@ -100,7 +100,7 @@ type action =
       (** Check whether the input files already are formatted. *)
   | Print_config of t  (** Print the configuration and exit. *)
 
-val action : action
+val action : unit -> action
 (** Formatting action: input type and source, and output destination. *)
 
 val debug : bool