Error_monad
Client_{node,proto}_rpcs
This refactors `src/node/shell/state.ml` in order to trace the source of blocks and operations. This prepares the node for the three-pass validator. In the procces, it adds an in-memory overlay for blocks and operations.
This required some modifications in the Base48 module, in order not to share the 'resolver' between distinct version of the economical protocol.