[WIP] Use concise JSON serialization for FrozenCircuit.

[95-martin-orion]

Fixes #3633. This PR applies the following changes:

• FrozenCircuit uses the efficient JSON serialization format.
• Users can optionally assign names to Circuit and FrozenCircuit objects. These have no special behavior for Circuits, but FrozenCircuits use their name and hash value as their serialization key.
• The JSON serialization test is now agnostic to hash values (0x + 16 hex characters) in serialization keys. This allows hardcoded JSON to match the JSON generated from code despite hash values not being consistent between Python sessions.

Note that since names are preserved across the Circuit/FrozenCircuit boundary, it is possible to e.g. call freeze() twice on the same Circuit, producing two FrozenCircuits with identical names. This is the desired behavior, as shown below:

• If the Circuit has not been mutated, the FrozenCircuits will have matching serialization keys and will be deduped in serialization.
• If the Circuit has been mutated, the FrozenCircuits' serialization keys will differ, and they will appear separately in serialization.

[95-martin-orion]

@Strilanc has pointed out that hashes should not be used for serialization. I'm leaving this PR up for reference, but if we continue with this serialization format I'll need to replace the hash with something else.

[95-martin-orion]

Experiments with a deeply nested circuit, defined as:

cs = [cirq.FrozenCircuit(cirq.H(a), cirq.CX(a, b))]
for i in range(1, 12):
    cs.append(cirq.FrozenCircuit(cs[i-1].to_op(), cs[i-1].to_op()))
test_circuit = cs[11]

show a distinct difference between compression times and sizes for gzip vs. this PR.

In the results below (collected from a single run), "nested" uses the circuit as described above, while "flattened" uses a decomposition of that circuit re-composed into a single-level circuit.

value	this PR	gzip
nested circuits	0.435255354 s	7.016707049 s
serialized size (bytes)	17500	299304
flattened circuit	2.14050902 s	1.278540381 s
serialized size (bytes)	4948515	20911

gzip code and benchmark can be seen in #3662; the benchmark for this PR is identical but with to_json/read_json in place of to_gzip/read_gzip.

[95-martin-orion]

Closing in favor of #3815.