Add a possibility to store pointers to inside of a trail.

[kzarzycki]

Storing such pointers would allow indexing long trails and start scanning the trail from the some middle-point.

From our private discussion with @tuulos :

Interesting idea. Storing the position requires both storing the bit-offset in the trail as well as the cursor state that is needed for decompression. We could consider adding serialize and deserialize functionality in cursors, so you could use frozen cursors to get back to a known location.