This commit removes the use of PolySpline for resampling curves and
replaces it with the length parameterization utility for that purpose.
That leaves only 5 remaining uses of the BKE_spline.hh header,
not including existing patches. I didn't test performance, but I
would expect the shrinking to be slightly faster because I reused
some arrays to avoid allocating them for every curve. I noted some
potential improvements in the "add curves" function.