Crate doesn't build due to changes to rustc

[tomprogrammer]

In AsciiCast impl unconstrained lifetimes appear which were forbidden. (rust-lang/rust#24461)

Feel free to submit a PR.

[tomprogrammer]

This commit also fixes a bug in the unsafe code of AsciiCast implementations!

Prior to v0.4.1 it was possible to move or mutable borrow a variable after it has been borrowed by AsciiCast::to_ascii(). For example the following code compiled successfully:

fn lifetime() {
    let mut text = "some text".to_string();

    let ascii = text.to_ascii().unwrap();
    text.clear();
    drop(text);
    println!("{}", ascii);
}

This code borrows text mutably and even frees it's memory afterwards. We can then access ascii without complaints by the compiler! That clearly violates memory safety! (Try unsafe { text.as_mut_vec()[1] = 'i' as u8 } and you will likely see it prints "sime text")

The cause of this behavior is found in AsciiCast and it's implementations. The lifetime of the returned &'a AsciiStr is unconstrained and can exceed the lifetime of it's origin as it is unsafely transmuted.

In v0.4.1 the lifetimes are connected correctly and compiling the given piece of code errors out early:

src/lib.rs:749:9: 749:13 error: cannot borrow `text` as mutable because it is also borrowed as immutable
src/lib.rs:749         text.clear();
                       ^~~~
src/lib.rs:748:21: 748:25 note: previous borrow of `text` occurs here; the immutable borrow prevents subsequent moves or mutable borrows of `text` until the borrow ends
src/lib.rs:748         let ascii = text.to_ascii().unwrap();
                                   ^~~~
src/lib.rs:752:6: 752:6 note: previous borrow ends here
src/lib.rs:745     fn lifetime() {
...
src/lib.rs:752     }
                   ^
src/lib.rs:750:14: 750:18 error: cannot move out of `text` because it is borrowed
src/lib.rs:750         drop(text);
                            ^~~~
src/lib.rs:748:21: 748:25 note: borrow of `text` occurs here
src/lib.rs:748         let ascii = text.to_ascii().unwrap();
                                   ^~~~

I've found this bug thanks to @nikomatsakis changes to prohibit unconstrained lifetimes that appear in associated types. Unconstrained lifetimes are mostly a bad idea I guess!