btree: Import toy B+ tree
btree: Import toy B+ tree

Use Amittai Aviram's toy B+ tree unmodified as a stopgap until a more
robust one is merged.

Signed-off-by: Ramkumar Ramachandra <5c355d1d38066ed0c5de420d67c1b5ed0687c4bd@gmail.com>

artagnon/phoenixfs,artagnon/phoenixfs,artagnon/phoenixfs