Add ability to convert from a hex string to Color32 using macros

[newcomb-luke]

This PR closes #1492.

Again currently ./sh/check.sh is currently failing for me, see my other PR.
Cargo clippy, cargo test, and cargo fmt have all been run.
The CHANGELOG.md has been updated under "Unreleased"

This change allows you to create Color32's from hex strings:

let color = epaint::hex_rgb!("#4c4c4c");

And it does so at compile-time.

[emilk]

Very nice!

But I would suggest we simplify it. As far as I can recall I've never seen premultiplied alpha expressed in hex form. So I suggest we boil it down to a single macro that accepts both RGB and unmultiplied RGBA (epaint::color_hex!("#55eedd") == epaint::color_hex!("#55eeddff")).

For additive, users can just do epaint::color_hex!("#ffeedd").additive() (we can make Color32::additive const fn)

[newcomb-luke]

Upon further testing, previously the from_rgb, from_rgb_additive, from_rgba_premultiplied were const, however from_rgba_unmultiplied due to the floating-point conversions that have to take place, is unable to be const yet. So only the conversion from the string literal to the array of values can be const. So the macro invocation itself cannot yet be const, but soon it could be. This isn't much slower than using the regular from_rgba_unmultiplied though.

[emilk]

It's a bit of a shame that it is not const but yeah, not much to do about that right now. I think this looks good, but the macro needs a docstring!

[coderedart]

is hex string assumed to be in linear space? or sRGB?
docs could mention that so its not confusing.
Color32 mention that Internally this uses 0-255 gamma space sRGBA color with premultiplied alpha. Alpha channel is in linear space..