Values of a transaction's inputs 0..upTo (inclusive), proven by the previous
transactions themselves: prevTxs[i] must hash to the txid named by input i.
prevTxs entries within the input count but beyond upTo are ignored, because
they cannot affect the position; entries past the input count are refused by
checkPrevTxCount before this function is reached.
Values of a transaction's inputs 0..upTo (inclusive), proven by the previous transactions themselves: prevTxs[i] must hash to the txid named by input i. prevTxs entries within the input count but beyond upTo are ignored, because they cannot affect the position; entries past the input count are refused by checkPrevTxCount before this function is reached.