Add Curve#d.localExtrema

[ianmackenzie]

Curve#d.localExtrema : Direction#d -> List Float would take a direction in which to minimize/maximize and then return a sorted list of parameter values that correspond to local minima or maxima with respect to that direction.

This could be useful for meshing (split curves into monotonic segments by finding extrema in X and/or Y directions), bounds calculation etc.