USe Irmin for the storage backend for tx_rollups
It passes the tests, and 3 runners are implemented:
- a runner for normal node operations (e.g. when you want to commit a transaction to disk)
- a runner to produce a proof for a given hash
- a runner to verify a proof
The code should move to its right place (right now it lives in the tests which is not great) and it depends on !4138 (merged) and on !3921 (closed)