[dml-code-generator] Complex Value Types now externalize to Json

  * Closes #165
[jvstm-ojb] Refactor: Now supporting complex ValueTypes

  * The backend now knows that complex Value Types are serialized to JsonElements,
    and as such, knows that the SQL type for them is that of the JsonElement.
  * Instead of doing manual [de]serialization of ValueTypes, the generated code
    now delegates that responsability to the ValueTypeSerializer
  * Closes #166