To be specific, support is added for `N op T<N>` where `N` is `f32` or `f64`,
`op` is one of `+` `-` `*`, and `T` is one of the `Vec`, `DVec`, `Mat`, or `DMat`
generic types. These are all cases where `T<N> op N` is already supported.
Rust does not support generic impls in this case, but `f32` and `f64` cover
many common cases.
Fixes#182.