Shiould add option to disable auto-wrapping of `Eigen::Ref<>`
See: https://github.com/RobotLocomotion/drake/pull/14265#pullrequestreview-616739166
For input arguments, I would prefer that we stick with const EigenType& x
for now, at least for Drake.
For return types, it is probably safest to keep it as by-value for now.