The index originally stored cumulative values in a CAmount type but this allowed for
potential overflow issues which were observed on Signet. Fix this by
storing the values that are in danger of overflowing in a arith_uint256.
Also turns an unnecessary copy into a reference in RevertBlock and
CustomAppend and gets
rid of the explicit total unspendable tracking which can be calculated
by adding the four categories of unspendables together.
431a076ae6