ddc-core-flow: support foldIndex and map2. change OpFold to take index as well, both OpFlowFold and OpFlowFoldIndex use OpFold
ddc-core-flow: make extract intersperse statements so that lets used by loops are defined at top etc
ddc-core-flow: DODGILY convert Nat# to Int#
ddc-core-flow: slurp map2