Order source map fields following the spec

Binaryen tools (up to version 123) expect this order.

Author: vouillon

compiler/lib/source_map.ml

@@ -477,9 +477,7 @@ module Standard = struct
            , match t.sourceroot with
              | None -> None
              | Some s -> Some (stringlit s) )
-         ; "names", Some (`List (List.map t.names ~f:(fun s -> stringlit s)))
          ; "sources", Some (`List (List.map t.sources ~f:(fun s -> stringlit s)))
-         ; "mappings", Some (stringlit (Mappings.to_string t.mappings))
          ; ( "sourcesContent"
            , match t.sources_content with
              | None -> None
@@ -489,6 +487,8 @@ module Standard = struct
                       (List.map l ~f:(function
                         | None -> `Null
                         | Some x -> Source_content.to_json x))) )
+         ; "names", Some (`List (List.map t.names ~f:(fun s -> stringlit s)))
+         ; "mappings", Some (stringlit (Mappings.to_string t.mappings))
          ; ( "ignoreList"
            , match t.ignore_list with
              | [] -> None

compiler/tests-jsoo/empty_sourcemap.t

@@ -8,13 +8,13 @@ Build object files and executable with --empty-sourcemap:
 
   $ dune exec -- js_of_ocaml --sourcemap --empty-sourcemap a.cmo -o a.js
   $ cat a.map
-  {"version":3,"file":"a.js","names":[],"sources":[],"mappings":"","sourcesContent":[]}
+  {"version":3,"file":"a.js","sources":[],"sourcesContent":[],"names":[],"mappings":""}
   $ dune exec -- js_of_ocaml --sourcemap --empty-sourcemap b.cmo -o b.js
   $ cat b.map
-  {"version":3,"file":"b.js","names":[],"sources":[],"mappings":"","sourcesContent":[]}
+  {"version":3,"file":"b.js","sources":[],"sourcesContent":[],"names":[],"mappings":""}
   $ dune exec -- js_of_ocaml --sourcemap --empty-sourcemap test.bc -o test.js
   $ cat test.map
-  {"version":3,"file":"test.js","names":[],"sources":[],"mappings":"","sourcesContent":[]}
+  {"version":3,"file":"test.js","sources":[],"sourcesContent":[],"names":[],"mappings":""}
 
 Build object files with sourcemap and link with --empty-sourcemap: