Add method to convert a Matrix into a Vec.

See https://github.com/rustsim/nalgebra/issues/473#issuecomment-440038489

The goal is to add an `.into_vec` method that would either unwrap the underlying `Vec` storage, or would perform a copy if the storage is not `MatrixVec`.

Note that one way of achieving this would be to add an implementation of `IntoIter` for `Matrix`. Then the `matrix.into_iter().collect()` will do the trick and will not perform any copy for matrices with a `MatrixVec` storage because of [that optimization](https://github.com/rust-lang/rust/issues/46084#issuecomment-345524508).