Residuals method on System trait

[pnevyk]

After return-position-impl-trait-in-trait feature gets into the stable, I would like to change the System trait to have the following API (some details omitted):

trait System: Problem {
    fn residuals<'a>(&self, x: &'a Vector) -> impl Iterator<Item = Self::Field> + 'a;

    fn eval(&self, x: &Vector, fx: &mut Vector) {
        // default impl, fill fx using self.residuals(x)
    }

    fn norm(&self, x: &Vector) -> Self::Field {
        // default impl, sqrt of sum of self.residuals(x)
    }
}

The main advantage is that by having residuals iterator the main interface, default implementation of norm can use that instead of eval and thus avoiding the need of allocating a temporary vector for fx. I believe that creating an iterator of residuals will not be more struggle than evaluating the system directly to fx. The iterator approach is already successfully used in the testing module.