Towards 0.7.0

[Ogeon]

For the sake of transparency and to not sit on all the plans, here are the main points I would like to do before making a version 0.7.0. It's also to make it easier if anyone would like to help out with moving it forward. I have been extra slow recently, but that's just because I have needed a break from programming on my free time. My energy for that comes and goes a bit.

I'll add anything I may have forgotten later, but these are pretty much the show stoppers for now.

Fixing up the traits

I have been refactoring and reworking the traits in the library to be more consistent and flexible. I'm also trying to remove default implementations (unless they are trivial and don't require any additional traits) in favor of blanket implementations where suitable. Or neither of them if not. This can break a lot of code so I want to get it all done in one release if possible.

The general pattern is like this:

• The default is to always move self and return Self. It's up to the user to clone their values if needed.
• An assigning companion trait can be added, that takes &mut self.
• Traits that take self are named after their main action or capability and has a single purpose, as much as possible. For example Lighten has a method lighten and do only that.
• Traits that take &mut self have the same name as their self counterparts but with an Assign suffix, and their methods have an _assign suffix. This is modeled after the std::ops traits, such as Add and AddAssign.
• There are two groups of traits. The ones described above modify the value as it is, while the other may extend it. They are named WithSomething for the self trait and SetSometing for the &mut self trait. For example WithAlpha can add transparency to an opaque color and SetAlpha can modify it.

An important goal, in addition to everything above, is that someone who want to use the trait in generics should only need a simple T: DoThing bound and not have to mention more of DoThing's dependencies than necessary.

Getting Okhsl, Okhsv and Okhwb in

They are currently sitting in #282 and need some fleshing out and brushing up before being merged. Not much to say, other than that I think it would be great to add them to the release.

Removing or deprecating Gradient

As discussed in #218, I would like to remove the gradient. It's pretty bad and adds to the maintenance burden. I'm also leaning more towards removing the named gradients too, instead of keeping the data. It makes more sense for an external crate to have them and many more.

The gradient is however quite popular so I may just deprecate it for this release.

Reducing the default feature set

Related to #286, I would like to make it less necessary to add default-features = false. The ideal would be if it can be reduced to only contain std.

[Ogeon]

A small change of plans. The traits and the default feature set will be released as they are for now. This is so the release isn't delayed for eternity. The other points are either in or on the way.