Feature request: Better conversions between Utf32String and Utf32Str

[bugnano]

For maximising performance of matching, I have a Vec<Utf32String> that I want to match against.
It's fairly big (hundreds of thousands of records).
For matching I do something like:

    let mut scores: Vec<(Utf32String, usize, u32, usize)> = entries
        .iter()
        .enumerate()
        .filter_map(|(i, file_name)| {
            let score = pattern.score(file_name.slice(..), matcher);

            score.map(|score| (file_name.clone(), i, score, file_name.len()))
        })
        .collect();

So that I can sort it like so:

    scores.sort_by(|(_file1, i1, score1, len1), (_file2, i2, score2, len2)| {
        score2.cmp(score1).then(len1.cmp(len2)).then(i1.cmp(i2))
    });

And finally, construct a new Vec<Utf32String> with the filtered result:

        scores
            .iter()
            .map(|(file_name, _i, _score, _len)| file_name.clone())
            .collect();

Notice that I do file_name.clone() twice.

Now, to avoid at least 1 clone, I was thinking of refactoring the scoring vector like so:

    let mut scores: Vec<(Utf32Str, usize, u32, usize)> = entries
        .iter()
        .enumerate()
        .filter_map(|(i, file_name)| {
            let slice = file_name.slice(..);
            let score = pattern.score(slice, matcher);

            score.map(|score| (slice, i, score, file_name.len()))
        })
        .collect();

So far so good, but then I found no way of converting an Utf32Str to Utf32String for the final Vec<Utf32String>

It would be nice to have, first of all an AsRef<Utf32Str> for Utf32String, so that I don't have to call .slice(..), next it would be awesome to have a From<Utf32Str> for Utf32String, so that I can do something like Utf32String::from(file_name) in the final Vec<Utf32String>