Move GateTabulation and friends to cirq-core as requested in #4461 (#4602)

- Adds `cirq. TwoQubitGateTabulation`, `cirq.TwoQubitGateTabulationResult`, and `cirq.two_qubit_gate_product_tabulation` public top level objects to Cirq. 
- TwoQubitGateTabulation is heuristic compilation technique for decomposing any 2q unitary to a given target 2q gate + single qubit rotations.

Author: Ashalynd

cirq-core/cirq/__init__.py

@@ -367,6 +367,9 @@
     two_qubit_matrix_to_diagonal_and_operations,
     two_qubit_matrix_to_operations,
     two_qubit_matrix_to_sqrt_iswap_operations,
+    two_qubit_gate_product_tabulation,
+    TwoQubitGateTabulation,
+    TwoQubitGateTabulationResult,
     unroll_circuit_op,
     unroll_circuit_op_greedy_earliest,
     unroll_circuit_op_greedy_frontier,

cirq-core/cirq/json_resolver_cache.py

@@ -155,6 +155,7 @@ def _parallel_gate_op(gate, qubits):
         'TaggedOperation': cirq.TaggedOperation,
         'TiltedSquareLattice': cirq.TiltedSquareLattice,
         'TrialResult': cirq.Result,  # keep support for Cirq < 0.11.
+        'TwoQubitGateTabulation': cirq.TwoQubitGateTabulation,
         '_UnconstrainedDevice': cirq.devices.unconstrained_device._UnconstrainedDevice,
         'VarianceStoppingCriteria': cirq.work.VarianceStoppingCriteria,
         'VirtualTag': cirq.VirtualTag,