Need a version with struct keys (custom comparator is provided)

[Pr0methean]

I'm trying to use a BigNumber library (https://github.com/Pr0methean/solidity-BigNumber/tree/0.5.0) to write a proof-of-concept for administering large-prime-number bounties. To do that efficiently, I need to maintain a sorted list of known primes (checking a primality certificate is a recursive process, so the list of known primes is for memoization as well as to prevent someone claiming the bounty twice for the same prime). I can't use OrderStatisticsTree to do that scalably, because the key type would need to be BigNumber.instance rather than uint. Could we please have that feature? I'll update OrderStatisticsTree for Solidity 0.5.x (which I also need) once that's done.