Updates to mini-btree library

The mini-btree library is here: https://github.com/tomjridge/mini-btree

Recently I made some changes. Now, mmap is used throughout, and there is no use of the Lwt monad. So, the interface is somewhat simpler, and the performance is slightly improved (although the performance was already fairly good, we can now achieve about 1M inserts per second). 

This work was done while employed by OCamlLabs, so many thanks to them. This library may in the future form part of the irmin/irmin-pack/index library used by Tezos.















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