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.
* SHA256 produces 256 bits pseudo-randomly uniformly, so you may
compare to a 256 bit target to get a proof of work
* If you pretend that the hash and targets are both integers between 0
and 2^256 - 1, then the target partitions the range into passing and
failing segments.
* In order to match the use of the `get_uint16` function from
`ocplib-endian`, the easiest way to encode `target` is as a `int list`
which works if not ideal
* This seems like the same thing bitcoin does; difficulty there is
actually not a primary notion but is calculated from a 256 bit target,
which is what gets adjusted over time
* include a public key in peer record type
* include public key in `Connect` packet
* receive/send public keys in `Connect` packet
* include public key in `connect_to_peer`
* json encode/decode public key
* include public key in peer file encode/decode
* include public key in bootstrapping
