The `begin_construction` function now accepts an optional argument
`proto_header`. This is to be used by a new RPC that ease forging the
shell header of a block (i.e. it will compute the fitness and, in a
near future, the hash of the resulting context).
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.
