Correctly handle "true" subtags in enum_keywords macro

[jedel1043]

This PR modifies unicode::Value and the enum_keyword macro to correctly handle true subtags. The added test has an extensive example of the (now fixed) incorrect behaviour.

I think it's important to note that this also slightly changes the semantics of Value itself; previously, something like

let v = Value::new_empty();
v.push_subtag(Some("subtag"));

would output the value subtag, while this PR makes it so that the sentinel subtag true is always considered when pushing a subtag, making the above output the value true-subtag. We could also specialize only enum_keyword to handle this correctly, but it seemed weird to have enum_keyword and Value differ in behaviour.

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[zbraniecki]

    /// let mut v = Value::default();
    /// v.push_subtag(subtag!("bar"));
    /// assert_eq!(v, "true-bar");

This does feel awkward. Why do we need this? Is that really the intention of LDML or just a letter? What is the value of this heuristic?

[jedel1043]

@zbraniecki I think we would have awkward situations either way, because something like this:

let mut v = Value::from_subtag(Some(subtag!("true")));
v.push_subtag(subtag!("bar"));
assert_eq!(v, "bar");

is also a bit awkward, so facilitating the implementation of enum_keyword seemed better overall.