update: slice syntax

The library doesn't compile with latest nightlies due to slice syntax introduction.
There's one thing that doesn't compile, and I'm both to tired now and not experieced enough to fix cryptography code, so I left a FIXME. If you can verify my code (which is a automatic transformation only) and help me with the last compile error, I would be very grateful. Thanks.
This line doen't compile with both "mut" and read-only borrow, compile tells me `cannot borrow immutable dereference of `&`-pointer as mutable` or `cannot borrow mutable dereference of `&`-pointer as mutable` depending on mutability operator I use.