Add unicode_ext_value! macro, enabled by new helper ShortVec

[sffc]

This PR introduces ShortVec, an internal class to icu_locid, which looks like this:

/// Internal: A vector that supports no-allocation, constant values if length 0 or 1.
pub(crate) enum ShortVec<T> {
    Empty,
    Single(T),
    Multi(Vec<T>),
}

With this type, I can swap out the internals of Value to use ShortVec, and then, with a little bit of gymnastics due to rust-lang/rust#73255, I can get unicode_ext_value! to work:

const CALENDAR_KEY: Key = unicode_ext_key!("ca");
const CALENDAR_VALUE: Value = unicode_ext_value!("buddhist");

If you like this direction, we could:

1. Keep it to just Value in Unicode and Transform
2. Change all Vec to ShortVec

If we did (2), then, with some gymnastics, we could actually make the following compile, which seems cool:

// Potential future if ShortVec is used everywhere
const LOCALE: Locale = locale!("en-Latn-US-macos-u-ca-buddhist-t-hi-h0-hybrid");

However, this is a little further off, because it would require replacing LiteMap again, or changing LiteMap itself to use ShortVec. I lean against this generalization for the time being since it is generally expected that there are multiple Unicode extension keywords, whereas with the values, they are often only one subtag long.

We actually could already do locale!("en-t-hi") without changing anything in the data model.

[sffc]

Note: Code size changes from 32072 to 32088. Seems like a worthwhile tradeoff.

[zbraniecki]

How generic you want this new type to be? If it is intended for subtags only, maybe SubtagList for a name?

[zbraniecki]

lgtm

[sffc]

How generic you want this new type to be? If it is intended for subtags only, maybe SubtagList for a name?

I don't want to take on the responsibility of exporting this type to be super generic, but I want it to be a bit more general than just SubtagList. I'm not far from making it be the LiteMap backend, for example.

[sffc]

Oops, I hit the merge button before @robertbastian reviewed 😞 Let me know if you have any feedback and I'll open a new PR for you.

[robertbastian]

lgtm