The minimal header now (classically) contains the root of a Merkle tree,
wrapping a list of lists of operations. Currently, the validator only
accept a single list of operations, but the 3+pass validator will
requires at least two lists.
- Let the `sandbox` change the public key
- Adapt `test_basic.ml` and `test_basic.sh`
- Do not duplicate the fitness in the block header
Client:
- allow the `bootstrap` command to lookup keys in the `head` rather than
in `genesis`.
