Add json serialization to sweeps

[dabacon]

Adds missing json serialization for sweep objects.

[95-martin-orion]

LGTM (% one concern, below). What is the expected use case for this?

[95-martin-orion]

Based on the name, _Unit is a private class. Do we want it to be serializable?

If so, I think this needs a _from_json_dict_ classmethod.

[dabacon]

This ends up being necessary in order to serialize UnitSweep. This is similar, I think to the Pauli singletons. My criteria was that with this PR it should serialize and deserialize cirq.UnitSweep correctly (should be UNIT_SWEEP btw), which it does. I think you can also use it to serialize and deserialize _Unit by going in and grabbing that class explicitly, but I don't know a way to stop this for this singleton pattern.

[95-martin-orion]

SGTM. This can merge once the tests are sorted out.

[dabacon]

Use case is that objects that set our context for experiments should be serializable.

[dabacon]

Ugh bad merge fixing

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