Preconvert in Color::parse to avoid codesize impact

[Manishearth]

Do not land.

We currently ascii-lowercase each time. We shouldn't do that in large matches.

We should probably have a COW-returning method instead of an unconditional allocation here; so that we retain the speed of not allocating in the common case.

cc @mbrubeck does this improve codesize?

r? @SimonSapin

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This change is 

[SimonSapin]

I don’t think repeatedly lowercasing is what hurts code size. (Though it might hurt runtime perf.) It’s rather generating code for each color keyword. I think having a static table and a loop that uses eq_ignore_ascii_case can also reduce code size. I’ll make a PR now.

[Manishearth]

Yeah, that's what dmajor suggested. I also wanted to try this because this may improve perf (once used with COW, that is)

[SimonSapin]

Here is the static table: https://github.com/servo/rust-cssparser/compare/color-keyword-static-table

I’ve written the Cow lower case thing before but didn’t end up using it. Here it is:

/// Like AsciiExt::to_ascii_lowercase, but avoids allocating when the input is already lower-case.
pub fn cow_into_ascii_lowercase<'a, S: Into<Cow<'a, str>>>(s: S) -> Cow<'a, str> {
    let mut cow = s.into();
    match cow.bytes().position(|byte| byte >= b'A' && byte <= b'Z') {
        Some(first_uppercase) => cow.to_mut()[first_uppercase..].make_ascii_lowercase(),
        None => {}
    }
    cow
}

[Manishearth]

Should we merge that branch instead?

[Manishearth]

r=me if you want to land it

[bors-servo]

☔ The latest upstream changes (presumably #118) made this pull request unmergeable. Please resolve the merge conflicts.