Suggest '.values_mut()' if `HashMap` mutably borrowed

[frewsxcv]

If one uses:

for (_, layer) in &mut self.layers {
    ...
}

clippy currently suggests to use HashMap::values. Since there is no HashMap::values_mut, there's no way to replicate this same functionality with a method, so it shouldn't be suggested.

[oli-obk]

there is a values_mut on HashMap since 1.10: https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.values_mut