Add no_std support to bevy_color

[bushrat011899]

Objective

• Contributes to Tracking Issue: MVP no_std Bevy #15460

Solution

• Added the following new features:
  • std (default)
  • alloc
  • encase (default)
  • libm

Testing

• Added to compile-check-no-std CI command

Notes

• ColorCurve requires alloc due to how the underlying EvenCore type works.
• Srgba::to_hex requires alloc to return a String.
• This was otherwise a very simple change

[TimJentzsch]

I have to say, I'm a bit surprised that stuff like rounding numbers is not available in core and has to be imported from bevy_math.

But the changes look fine to me :)

[bushrat011899]

I have to say, I'm a bit surprised that stuff like rounding numbers is not available in core and has to be imported from bevy_math.

Yeah they were in core in early versions of Rust, but have since been removed. The reason was they depended on libm for their implementation, and they wanted core to be as slim as possible. Since libm is also a no_std library with pretty much perfect platform support it isn't really that big of a change. I'd previously even discussed the idea of adding an extension trait in bevy_math that would make this change entirely transparent. The idea was eventually scrapped, as it's probably a good thing to highlight which maths operations are "problematic".