Remove leading and trailing whitespace

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• Read more about trim and the related functions.

examples/strings/trim/src/main.rs

fn main() {
    let text = String::from("  Some text  ");
    println!("original:   '{}'", text);
    println!("trim_end:   '{}'", text.trim_end());
    println!("trim_start: '{}'", text.trim_start());
    println!("trim:       '{}'", text.trim());
}

original:   '  Some text  '
trim_end:   '  Some text'
trim_start: 'Some text  '
trim:       'Some text'