Add explicit JSON resolver function in contrib

[KevinVillela]

Fixes #2445

This PR adds an explicit function to return a JSON serializer containing Contrib classes. Right now, only QuantumVolumeResult is JSONable, but in the future other classes can be added here as well. Users who want to use the JSON family of functions will need to explicitly pass the contrib_class_resolver, as is done in the test.

Additionally, this pulls out a useful json_test function into the testing directory, for use elsewhere.

[Strilanc]

This name is too vague; there is not enough context to know what cirq.testing.assert_roundtrip refers to.

Rename this to something more explicit like assert_json_roundtrip_works.

We may want to include this into the standard test battery for gates (assert_implements_consistent_protocols).

[KevinVillela]

Fixed the name, thanks!

I tried adding this to assert_implements_consistent_protocols, but that function is called with a ton of test gates (e.g. GateUsingWorkspaceForApplyUnitary) :\

[Strilanc]

Leftover debug line

[KevinVillela]

D'oh I fixed that and forgot to commit it :P Done.

[KevinVillela]

Friendly ping :)

[mpharrigan]

this looks good to me

[mpharrigan]

you could add

DEFAULT_RESOLVERS = [contrib_class_resolver] + protocols.json.DEFAULT_RESOLVERS

then

import cirq
import cirq.contrib
data = cirq.read_json(fn, resolvers=cirq.contrib.DEFAULT_RESOLVERS)

The above relies on python namespacing for disambiguation. You could make the name more specific: DEFAULT_CONTRIB_RESOLVERS if you want

[KevinVillela]

Good idea, done and used in the test.

[mpharrigan]

@Strilanc you good?

[CirqBot]

Automerge cancelled: Need a fresh 🍪.