2dba8607 (go/zodb/btree: New package to work with ZODB BTrees (draft))
added btree module with btree.BTree essentially being LOBTree (int64 key
-> object). Since for wendelin.core we also need IOBTree (int32 key ->
object), which is used in ZBlk1

	https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.12-6-g318efce/bigfile/file_zodb.py#L267
	https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.12-6-g318efce/bigfile/file_zodb.py#L374

let's turn btree module into template internally and generate code for
both LOBTree and IOBTree.

For the reference BTree/py takes similar approach with respect to
templating.