Simplify flatten

No longer use it for internally-tagged enums. Instead, use a private helper that adds the tag property.

schemars/tests/expected/enum-internal-duf.json

@@ -112,10 +112,7 @@
       "additionalProperties": false
     },
     {
-      "type": [
-        "object",
-        "integer"
-      ],
+      "type": "object",
       "format": "int32",
       "required": [
         "typeProperty"

schemars/tests/expected/enum-internal.json

@@ -28,6 +28,9 @@
             "StringMap"
           ]
         }
+      },
+      "additionalProperties": {
+        "type": "string"
       }
     },
     {
@@ -105,10 +108,7 @@
       }
     },
     {
-      "type": [
-        "object",
-        "integer"
-      ],
+      "type": "object",
       "format": "int32",
       "required": [
         "typeProperty"

schemars/tests/expected/schema_with-enum-internal.json

@@ -21,10 +21,7 @@
       }
     },
     {
-      "type": [
-        "object",
-        "boolean"
-      ],
+      "type": "object",
       "required": [
         "typeProperty"
       ],
@@ -38,10 +35,7 @@
       }
     },
     {
-      "type": [
-        "object",
-        "boolean"
-      ],
+      "type": "object",
       "required": [
         "typeProperty"
       ],