Stdio.IOBuffer: bad return type of add_int32()
Added Stdio.IOBuffer::rewind_on_error.

Use this function to cause the buffer to be rewound to the location
it was at when the function was called if the object it returs
goes out of scope.
Stdio.IOBuffer.UnwindKey: allow explicit ->rewind()
Survive the SSL.File read callback closing the file.
Added read_hint and add_hint

They read and add integers encoded like hstrings, first a length then the number.
Added rewind_key, much like rewind_on_error, but not automatic.

Also speed up read_cstring about a factor of two.
SSL.Cipher: Improved debug message.