Implement support for enum fields in JSON macro.

tests/stdlib/tjsonmacro.nim

@@ -166,4 +166,20 @@ when isMainModule:
     var data = to(jsonNode, Data)
     doAssert data.person.name == "Nimmer"
     doAssert data.person.age == 21
-    doAssert data.list == @[1, 2, 3, 4]
+    doAssert data.list == @[1, 2, 3, 4]
+
+  # Test non-variant enum fields.
+  block:
+    type
+      EnumType = enum
+        Foo, Bar
+
+      TestEnum = object
+        field: EnumType
+
+    var node = %{
+      "field": %"Bar"
+    }
+
+    var result = to(node, TestEnum)
+    doAssert result.field == Bar