Intead of providing a `fitness` function, an economic protocol should
now call `Context.set_fitness`.
This simplify the shell's code and avoid complexity on protocol
change. Previously the fitness of a context produced by the old protocol
had to be read by the new protocol. Now, the shell read the context
without requesting the help of the economic protocol.
- 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`.
