update `*PublicKey.Verify` to use `ristretto255.VarTimeDoubleScalarBaseMult`

the following lines in `*PublicKey.Verify` can be simplified:
```
Rp := r255.NewElement()
Rp = Rp.ScalarBaseMult(s.s)
ky := r255.NewElement().ScalarMult(k, p.key)
Rp = Rp.Subtract(Rp, ky)
```

to:
```
Rp := r255.NewElement().VarTimeDoubleScalarBaseMult(k, r255.NewElement().Negate(p.key), s.s)
```
see #40