Use static duals (`DualSVec64`) instead of dynamic (`DualDVec64`)

[ryan-williams]

Dual in this repo wraps a num_dual::DualDVec64, which is dynamically sized / wraps a Vec.

It would be better to wrap DualSVec64, which implements Copy, and would allow for removing a lot of .clone()s that currently pollute the math here (example).