In the next patch we are going to expose access to BTree/Bucket entries
as public API. This will turn _BTreeItem into Entry and will also add
BucketEntry data type. Before doing that rearrange the order in which
the data structure go:

	- BTree,
	- Entry (_BTreeItem for now),
	- Bucket
	- BucketEntry (not present for now).

Only code movement - no other change.