Deprecate, Remove or Enhance toDict Serialization in JSON
Right now we provide custom serialization for JSON objects that define a toDict method:

https://pandas.pydata.org/pandas-docs/stable/user_guide/io.html?highlight=todict#fallback-behavior

Some notes about this:

 - toDict was a method internally for Series and Frame that was deprecated at least as far back as v0.4
 - There is one test (test_to_dict) in test_ujson that this can be encoded / decoded, but nothing comprehensive or user facing
 - It is not the suggested approach for serializing custom objects (default_handler is instead)

I'm trying to simplify the JSON extension that we have internally so I would really like to remove this as it seems poorly tested and very unlikely to be used given sparse documentation, deprecated naming convention and unlikeliness that all objects in a container would have toDict defined.