fixes#4695
ref https://github.com/nim-lang/Nim/pull/15818
Since `nkState` is only for the main loop state labels and `nkGotoState`
is used only for dispatching the `:state` (since
https://github.com/nim-lang/Nim/pull/7770), it's feasible to rewrite the
loop body into a single case-based dispatcher, which enables support for
JS, VM backend. `nkState` Node is replaced by a label and Node pair and
`nkGotoState` is only used for intermediary processing. Backends only
need to implement `nkBreakState` and `closureIterSetupExc` to support
closure iterators.
pending https://github.com/nim-lang/Nim/pull/23484
<del> I also observed some performance boost for C backend in the
release mode (not in the danger mode though, I suppose the old
implementation is optimized into computed goto in the danger mode)
</del>
allPathsAsgnResult???
