We want to replace the remaining usage of big_number which is used in a
few places for calculating work (comparisons, basic arithmetic).

I'm thinking to copy uint256.h over from bitcoind:

https://github.com/bitcoin/bitcoin/blob/master/src/uint256.h

to include/bitcoin/utility/external/

and then make a custom wrapper in include/bitcoin/utility/
which implements a subset of that class which we require.

Probably called 'hash_number' which is a descriptive name of this class.
Another possibility is pow_number (pow = proof of work, not prisoner...)
Or even proof_of_work_number?

#include <bitcoin/utility/hash_number.hpp>

Then after it's a few small steps to remove elliptic_curve_key,