Rollup merge of #88858 - spektom:to_lower_upper_rev, r=dtolnay

Allow reverse iteration of lowercase'd/uppercase'd chars

The PR implements `DoubleEndedIterator` trait for `ToLowercase` and `ToUppercase`.

This enables reverse iteration of lowercase/uppercase variants of character sequences.
One of use cases:  determining whether a char sequence is a suffix of another one.

Example:

```rust
fn endswith_ignore_case(s1: &str, s2: &str) -> bool {
    for eob in s1
        .chars()
        .flat_map(|c| c.to_lowercase())
        .rev()
        .zip_longest(s2.chars().flat_map(|c| c.to_lowercase()).rev())
    {
        match eob {
            EitherOrBoth::Both(c1, c2) => {
                if c1 != c2 {
                    return false;
                }
            }
            EitherOrBoth::Left(_) => return true,
            EitherOrBoth::Right(_) => return false,
        }
    }
    true
}
```

0 files changed, 0 insertions, 0 deletions