Enumerator.Prev() would return the next item if there was no hit.
This fix instead changes Prev to return previous item as expected.
Resolving issue https://github.com/cznic/b/issues/6.

Fix slice out of bounds crash

There was crash when we try insert element to full data struct at position i=0
when left neighbour data struct was not full

Fix example signature for Put explanation

clarify documentation