Nicer string representation for InverseCompositeGate (quantumlib#6262)

Change `cirq.inverse(g)` string representation to `g.__str__()†` instead of `(g.__repr__() ** -1)`.

cirq/ops/raw_types.py

@@ -1032,6 +1032,9 @@ def _circuit_diagram_info_(self, args: 'cirq.CircuitDiagramInfoArgs'):
     def __repr__(self) -> str:
         return f'({self._original!r}**-1)'
 
+    def __str__(self) -> str:
+        return f'{self._original!s}†'
+
 
 def _validate_qid_shape(val: Any, qubits: Sequence['cirq.Qid']) -> None:
     """Helper function to validate qubits for gates and operations.

cirq/ops/raw_types_test.py

@@ -759,6 +759,7 @@ def __repr__(self):
     assert cirq.parameter_names(g) == {'a'}
     assert cirq.resolve_parameters(g, {a: 0}) == Gate(0) ** -1
     cirq.testing.assert_implements_consistent_protocols(g, global_vals={'C': Gate, 'a': a})
+    assert str(g) == 'C(a)†'
 
 
 def test_tagged_act_on():