Optionally support serde.

[waywardmonkeys]

This adds support for serde::Deserialize and serde::Serialize using serde-derive for AlphaColor, OpaqueColor, PremulColor, Rgba8.

This does not provide it for DynamicColor to avoid dealing with the non-exhaustive ColorSpaceTag.

It uses derive rather than hand-implementing the traits as the latter is very verbose and most things using serde will already have enabled the derive feature. (This is also true for the other Linebender crates.)

[waywardmonkeys]

The fact that this uses the derive feature for serde doesn't change the decision to not use it for bytemuck.

In particular, crates like resvg, tiny-skia, and those from Makepad do use bytemuck, don't use serde and very much do not want a dependency on the whole proc macro world that serde-derive builds on.

[raphlinus]

Looks straightforward, and I agree with the decision not to hand-derive. People who use serde are opting in to that whole thing.

My personal feeling is that if we're annotating these types with serde, then DynamicColor belongs also, but I understand the concern about non-exhaustive and can defer that discussion to later.