Add Color::hex fn

[wyhaya]

Allows to create color from hex value

// RGB
Color::hex("FFF").unwrap() == Color::rgb(1.0, 1.0, 1.0)
// RGBA
Color::hex("FFFF").unwrap() == Color::rgba(1.0, 1.0, 1.0, 1.0)
// RRGGBB
Color::hex("FFFFFF").unwrap() == Color::rgb(1.0, 1.0, 1.0)
// RRGGBBAA
Color::hex("FFFFFFFF").unwrap() == Color::rgba(1.0, 1.0, 1.0, 1.0)

[ncallaway]

Does bevy have a minimum rust version that's targeted, or is latest stable OK? With the new improvements to const fn that came out today (https://blog.rust-lang.org/2020/08/27/Rust-1.46.0.html#const-fn-improvements) I'm pretty sure we could convert this to a const fn, which would be awesome

[cart]

I haven't quite sorted out how I want to handle minimum rust versions. Maybe whatever is currently in stable ubuntu? For now, that feature is fresh enough that I think we should hold off.

[zicklag]

What if we had a feature flag for optimizations that are only available on later versions of Rust? It would be disabled by default, but could be enabled to take advantage of later features?

[ncallaway]

I tried playing with it and think even with 1.46 there's not enough str manipulation to easily create a const fn for creating a Color from a hex string.

I think the feature flags for newer rust optimizations is a good idea for optional functionality, but I don't think this will be a place we can add it (sadly)

[cart]

I think in general I would prefer to keep the features as minimal as possible. We can just adopt adopt new features when they hit our MSRV (whatever that ends up being).

Each code path we add introduces maintenance burden and build complexity. I'd rather just support one.

[cart]

Awesome work! Just resolve merge conflicts and we're good to go!

(sorry for merging a newer pr first + creating the conflicts. i'll try to give precedence to earlier prs in the future)

[CleanCut]

Dependency check:

• 1 new dependency: hex
  • hex's only runtime dependency is a off-by-default optional serde dependency.

[cart]

Great work!