Added in the compressed length field to the rANS codec header.

Technically this isn't needed, but it adds a very small extra amount
of space and it allows for the internal rANS codec to be the same
format as an external block-based rANS codec, or to allow the rANS
codec internally to compress very large buffers using a smaller block
size.

This now brings this code into line with the Java cramtools