Remove use of CBigNum from GetBlockTrust()
Remove use of CBigNum from GetBlockTrust()

This adopts Peercoin's change to CBlockIndex::GetBlockTrust() to use
Bitcoin's arith_uint256 type for big-integer arithmetic to avoid the
conversion from the old CBigNum type.

gridcoin/Gridcoin-Research,caraka/gridcoinresearch,caraka/gridcoinresearch,gridcoin/Gridcoin-Research,gridcoin/Gridcoin-Research,caraka/gridcoinresearch,caraka/gridcoinresearch,gridcoin/Gridcoin-Research,Git-Jiro/Gridcoin-Research,Git-Jiro/Gridcoin-Research,caraka/gridcoinresearch,gridcoin/Gridcoin-Research,Git-Jiro/Gridcoin-Research,caraka/gridcoinresearch,gridcoin/Gridcoin-Research,Git-Jiro/Gridcoin-Research,Git-Jiro/Gridcoin-Research