Function set_variations 

fn set_variations<V>(
    fvar: &Fvar<'_>,
    avar: Option<Avar<'_>>,
    coords: &mut CoordStorage,
    variations: V,
)
where
    V: IntoIterator,
    V::Item: Into<FontVariation>,

Helper for setting variations.

Pulled out into a separate function to avoid borrow checker issues.