JSON backwards compatibility has been flushed

[mpharrigan]

#4099 cleared some deprecated classes. Serialized files have a much longer lifespan than code. It was always the intent to keep much longer-term backwards compatibility for data files, especially experimental results which likely have cirq.TrialResult classes.

1. the old TrialResult.json should be kept around like the "inward" files
2. The resolver should re-direct "TrialResult" keys to cirq.Result. In some sense, this should have been done along with the initial rename.

[dstrain115]

Oops, let's fix this ASAP.