encoding/openapi: make OrderedMap type opaque

The OrderedMap needs to be a pointer, which may be
a bit unintuitive it being a slice. Change it to as struct.

Info is left as a non-pointer to indicate the non-optional
nature of it. The user of pointers is enforced by making
MarshalJSON only work for pointer types.

Change-Id: I8481dd8974825928815bfef3acd1eb5fc0274029
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2460
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/testdata/oneof.json b/encoding/openapi/testdata/oneof.json
index 83384fb..f37e664 100644
--- a/encoding/openapi/testdata/oneof.json
+++ b/encoding/openapi/testdata/oneof.json
@@ -42,8 +42,8 @@
                "count"
             ],
             "properties": {
-               "count": {
-                  "$ref": "#/components/schema/MyInt"
+               "include": {
+                  "$ref": "#/components/schema/MyString"
                },
                "exclude": {
                   "type": "array",
@@ -51,8 +51,8 @@
                      "$ref": "#/components/schema/MyString"
                   }
                },
-               "include": {
-                  "$ref": "#/components/schema/MyString"
+               "count": {
+                  "$ref": "#/components/schema/MyInt"
                }
             }
          }