Colour Structs

[martindevans]

This PR proposes a breaking change.

Currently, this library causes lots of unavoidable allocations. That makes it unusable when developing games, where performance is critical and allocations should be avoided as much as possible.

To fix this, I've converted all of the colour structs from class to readonly record struct.

An added benefit of this change is removing all of the handwritten equality implementations (we get it for free from record), which were broken! For example this would previously crash instead of return false:

var rgb = new RGB(1, 2, 3);
var hsl = new HSL(1, 2, 3);
Assert.IsFalse(rgb.Equals(hsl));