Migration to the new API can be achieved with this rsc.io/rf script:
```
rf ex {
import "gonum.org/v1/gonum/spatial/r2";
var p,q r2.Vec;
var f float64;
p.Add(q) -> r2.Add(p, q);
p.Sub(q) -> r2.Sub(p, q);
p.Scale(f) -> r2.Scale(f, p);
p.Dot(q) -> r2.Dot(p, q);
p.Cross(q) -> r2.Cross(p, q);
p.Rotate(f, q) -> r2.Rotate(p, f, q);
}
```
Updates gonum/gonum#1553.
