Use `where` to make code more readable

[conradoplg]

We can use where to make some code more readable, mainly batch.rs

For example:

impl<S: SpendAuth, B: Binding<Scalar = S::Scalar, Point = S::Point>> Default for Verifier<S, B> {

could be

impl<S, B> Default for Verifier<S, B>
where
    S: SpendAuth,
    B: Binding<Scalar = S::Scalar, Point = S::Point>,
{

Improve this and any other similar instances, if any.