Modification of standard Rust library.

Example:

#[macro_use] extern crate std_helper;
use std_helper::StrHelper;

fn str_helper_example() -> std::io::Result<()> {
    let mut helper = str!("Hello, Rust!");

    let result = helper.remove("Rust").unwrap();
    assert_eq!(result, "Hello, !");

    helper.push("World");
    assert_eq!(helper.as_str(), "Hello, !World");

    helper.move_to_the_end((7, 8));
    assert_eq!(helper.as_str(), "Hello, World!");

    Ok(())
}

str_helper_example().unwrap();