make SingleQubitCliffordGate immutable singletons and use it in qubit_characterizations for a 37% speedup

SingleQubitCliffordGates represents the 24 gates in the single qubit clifford group. Some of the operations this class implements are expensive computation but at the same time are properties of each of the 24 operators.

turning those expensive computations into cached properties is benificial for performance but for that the objects need to be immutable.

one of the places that heavily relies on single qubit cliffords is qubit_characterizations.py which implemented the single qubit clifford algebra from scratch by falling on to the matrix representation and doing matrix multiplication and inversion (for computing the adjoint) this led to a bottleneck while creating circuits (e.g. for example _create_parallel_rb_circuit for 50 qubits and a 1000 gates takes $3.886s$). using SingleQubitCliffordGates instead and using merged_with operation which maps two cliffords onto the clifford equaivalent to their composition leads to $2.148s$, with most of those 2s spent in Moment.__init__ which will be the target of the next PR.

I also made the 24 cliffords singleton, since there is no point in creating new object which won't have any of the cached properties.

part of https://github.com/quantumlib/Cirq/issues/6389

quantumlib / Cirq

make SingleQubitCliffordGate immutable singletons and use it in qubit_characterizations for a 37% speedup #6392

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