Make rustworkx the default backend

[RMeli]

The performance of spyrmsd seems to have been mis-represented in a recent pre-print:

• Canonized then Minimized RMSD for Three-Dimensional Structures
• https://github.com/JerryJohnsonLee/CanonizedRMSD/issues/8

This is likely due to the poor performance of networkx, which is currently the default backend.

Make rustworkx the default backend and make a new 0.8.0 release.