AbstractCircuits and FrozenCircuits (#3481)

Partial reupload of #3445, following the method reordering submitted in #3478.

Commit history has been cleaned up to remove a misconfigured YAPF run from the previous upload. The following items have also been addressed:

@balopat 
> There are no tests protecting immutability for FrozenCircuit - I can just add a the method below to FrozenCircuit - making it mutable, and there are no tests broken.

Tests have been added to ensure `moments` and `devices` cannot be set with `=`.

@maffoo 
> [frozen_circuit_test.py] is almost entirely duplicated from {...} circuit_test.py. It seems like a better way to handle this would be to write a single test that is parameterized over whether to use a frozen circuit, e.g. {...}

Shared tests have been converted to parameterized tests in `circuit_test.py`. `frozen_circuit_test.py` now only contains tests for FrozenCircuit-specific behavior.

cirq/__init__.py

@@ -66,6 +66,7 @@
 from cirq.circuits import (
     Circuit,
     CircuitDag,
+    FrozenCircuit,
     InsertStrategy,
     PointOptimizationSummary,
     PointOptimizer,

cirq/circuits/__init__.py

@@ -24,11 +24,15 @@
     QuilOutput,)
 
 from cirq.circuits.circuit import (
-    Circuit,)
+    AbstractCircuit,
+    Circuit,
+)
 from cirq.circuits.circuit_dag import (
     CircuitDag,
     Unique,
 )
+from cirq.circuits.frozen_circuit import (
+    FrozenCircuit,)
 from cirq.circuits.insert_strategy import (
     InsertStrategy,)