Make bytes!() return 'static

Change `bytes!()` to return

    {
        static BYTES: &'static [u8] = &[...];
        BYTES
    }

This gives it the `'static` lifetime, whereas before it had an rvalue
lifetime. Until recently this would have prevented assigning `bytes!()`
to a static, as in

    static FOO: &'static [u8] = bytes!(1,2,3);

but #14183 fixed it so blocks are now allowed in constant expressions
(with restrictions).

Fixes #11641.