This commit migrates the two functions in the private module
`qiskit.synthesis.two_qubit.local_invariance` to primarily be
implemented in rust. Since the two_qubit_local_invariants() function is
being used in Qiskit#12727 premptively porting these functions to rust will
both potentially speed up the new transpiler pass in that PR and also
facilitate a future porting of that pass to rust. The two python space
functions continue to exist as a small wrapper to do input type
checking/conversion and rounding of the result (since the python API for
rounding is simpler). There is no release note included for these
functions as they are internal utilities in Qiskit and not exposed as a
public interface.