impl DoubleEndedIterator for {MatrixIter, MatrixIterMut}

Currently `vec.iter().rev()` fails because [`DoubleEndedIterator`](https://doc.rust-lang.org/stable/core/iter/trait.DoubleEndedIterator.html) is not implemented for [`MatrixIter`](https://docs.rs/nalgebra/0.23.0/nalgebra/base/iter/struct.MatrixIter.html). By a quick layman's glance at [the source](https://docs.rs/nalgebra/0.23.0/src/nalgebra/base/iter.rs.html#13-20) it seems straightforward to implement, but please tell me what I'm missing before I attempt this.

My usecase is Horner evaluating a coefficient vector. There are plenty workarounds, but `iter().rev()` seems the most natural to me.