Better support for multivariate distributions

[vks]

The Distribution trait does not work well for multivariate distributions, because impl Distribution<Vec<f64>> for Multivariate requires an allocation for every sample (see #485 for an example). Maybe we should add another trait:

pub trait MultivariateDistribution<T> {                                                         
    fn sample_multi<R: Rng + ?Sized>(&self, rng: &mut R, buffer: &mut [T]);
}

With SIMD in mind, this might even make sense for univariate distribution. So we might want to add sample_multi to the Distribution trait.

[dhardy]

This is a good point. My preference is to postpone this until we resolve #290 or have more multi-variate distributions.

As mentioned in #485 there might be another solution, if fixing the number of variables at compile-time is sufficient: using generic consts (rust-lang/rust#44580).

[vks]

Nice, I did not know const generics were so close!

rust-lang/rust#51192 (comment)

Given that const generics on nightly might be only a month away, my vote would be to wait...