BTree: Add overrides for prefix/suffix family of functions.
BTree: Add overrides for prefix/suffix family of functions.

Also add key-based prefix/suffix functions.

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