Convert BTreeNode._order and _depth into let constants.
Convert BTreeNode._order and _depth into let constants.

They’re never supposed to be mutated anyway.

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