Add support for rotations, insertions and access to payload & head.
Add support for rotations, insertions and access to payload & head.

(Entirely untested so far.)

Maaimusic/BTree,lorentey/BTree,lorentey/BTree,eonil/BTree,Maaimusic/BTree,Maaimusic/BTree,lorentey/BTree,eonil/BTree,eonil/BTree