Minimal CProof class independent from pow.o
CheckProofOfWork(nBits) -> CheckProof(CProof)
SQUASHME: CProof in miner.cpp
Use __func__ in error messages
pow.o depends on chain.o, but not main.o
Implement CProof::ToString()
Implement GetChallengeDouble(CProof) (Move 2 direct accesses to
proof.nBits to pow)
Hide GetNextWorkRequired()
GetProofIncrement(nBits) -> GetProofIncrement(CProof)
MOVEONLY: Move void UpdateTime() from pow.o to miner.o