Use T.new(value) to typecast to generic index types, not T.zero + value

[shinzlet]

This is mostly a pattern that's used in IndexRegion and (i think) the iterator classes - often we need to get the value 1 in some generic integer type T, so we've been using T.zero + 1 to typecast. But you can also just do T.new(1), which is cleaner, more obvious, and safer overall.