Refactoring, better ability to use cssparser-color as a separate crate.

[chipnertkj]

• Use case for better cssparser-color separation:
  Access to private implementation items for parsers and other software using the crate.
• Example use case and motivation:
  My own project. I use the crate to serialize and deserialize CSS3-compatible color data from user input and config files. This was impossible without these changes.
• Serde impls for color crate items are gated by a new serde feature in the inner crate.
• A few public functions were adjusted with new names, as suggested by clippy - caused a semantic conflict with std's Iterator.
• General refactoring.

All checks passed on my fork, but I'm not familiar with the guidelines for commits for this repository. It's my first one here. I hope it's up to expectations. 😁
If something's up, be sure to comment.

[chipnertkj]

Apparently part of the changes were also requested here: #376

[emilio]

A lot of these look good but can we avoid the breaking changes in cssparser? Seem unnecessary to me unless I'm missing something.

[chipnertkj]

@emilio That's absolutely doable, it's just that I see valid reasoning in the clippy hint, enough of it to increase the minor version, especially since the project isn't in a mature state yet. Realistically, would that change be problematic? Genuine question.

[emilio]

Yeah, the tokenizer uses the try prefix (well, now try_parse) for stuff that rewinds, so try_next doesn't feel necessarily like a great name. Also, in a sense the tokenizer / parser's next() is basically an Iterator (over the tokens), even if it returns an extra info for the errors.

It'd also mean we'd have to rewrite quite a few callers. That's not a huge deal if the improvement is clear, but for me at least try_next is just a longer and more confusing name...

In any case, those changes should probably be their own PR. I'd prefer to split the different high-level changes in different PRs if that's possible, so that they can be reviewed / merged independently.

[chipnertkj]

I see, sure thing! I'll revert the naming and version changes as soon as possible.

[chipnertkj]

Okay, that should be it.

[emilio]

Thanks, this looks great, but the commit history is a bit of a mess. I sent #378 with it cleaned-up.