Trim methods on slices

/// Trims this slice from the left.
fn trim_left_matches<F: Fn(T) -> bool>(&self, f: F) -> &[T] {
    let mut res = self;
    while res.len() > 0 && f(res[0]) {
        res = res[1..];
    }
    res
}

/// Trims this slice from the right.
fn trim_right_matches<F: Fn(T) -> bool>(&self, f: F) -> &[T] {
    let mut res = self;
    while res.len() > 0 && f(res[res.len()-1]) {
        res = res[..(res.len()-1)];
    }
    res
}

(and so on)

basically turns &["", "", "", "foo", ""] into &["foo", ""], &["", "foo", "", "", ""] into &["", "foo"], etc, depending on what you call.

Start with the issue body and its proposed slice-trimming examples; no implementation files, tests, or entry points are identified in the payload. Clarify the full method scope and expected behavior for each direction before locating the Rust slice API, and consider the work done when the agreed methods support the stated examples.