Provide an Irmin store based on a binary tree in [lib_context]

Right now, lib_context provide a Irmin store with a 32-ary tree. In the long run, there is a plan to move to a binary tree with more a efficient/less ad-hoc configuration.

Besides, layer-2 solutions such as Transaction Rollups will make use of such a store very soon.

As a consequence, we believe the best way forward is to modify lib_context to provide both stores. That could allow for migrating Tezos context at some point.