* 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
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