std: Use correct lifetime parameter on str::raw::slice_bytes

fn slice_bytes is marked unsafe since it allows violating the valid
string encoding property; but the function did also allow extending the
lifetime of the slice by mistake, since it's returning `&str`.

Use the annotation `slice_bytes<'a>(&'a str, ...) -> &'a str` so
that all uses of slice_bytes are region checked correctly.
auto merge of #8308 : blake2-ppc/rust/str-slice-bytes, r=alexcrichton

`fn slice_bytes` is marked unsafe since it allows violating the valid
string encoding property; but the function did also allow extending the
lifetime of the slice by mistake, since it's returning `&str`.

Use the annotation `slice_bytes<'a>(&'a str, ...) -> &'a str` so
that all uses of `slice_bytes` are region checked correctly.