spatial/r3: deprecate Skew package level function in favor of method

This is preferred to prevent heap allocations.

spatial/r3/mat_safe.go

@@ -60,6 +60,8 @@ func Eye() *Mat {
 //                  ⎡ 0 -z  y⎤
 //  Skew({x,y,z}) = ⎢ z  0 -x⎥
 //                  ⎣-y  x  0⎦
+//
+// DEPRECATED: use Mat.Skew()
 func Skew(v Vec) (M *Mat) {
 	return &Mat{&array{
 		0, -v.Z, v.Y,