fixes #4695; closure iterators support for JS backend

[ringabout]

fixes #4695

ref #15818

Since nkState is only for the main loop state labels and nkGotoState is used only for dispatching the :state (since #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 #23484

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)

allPathsAsgnResult???

[zerbina]

There's a small error in the PR description: nkBreakState, not nkGotoState, has to be implemented by the code generators in order to support closure iterators.

[ringabout]

import asyncdispatch

proc main {.async.} =
  proc foo: Future[int] {.async.} =
    echo 1
  let x = await foo()

waitFor(main())

expands into previously

goto :state 1
var :state
state 0:

Now:

var :state
case :state
of 0:

I suppose :state is not lifted correctly

[ringabout]

check: tests/async/tasynctry.nim

[ringabout]

read(cast[typeof(problemCall(
  template yieldFuture() {.redefine.} =
    yield FutureBase()

  var internalTmpFuture`gensym30: FutureBase = expandValue()
  yield internalTmpFuture`gensym30
  read(cast[typeof(expandValue())](internalTmpFuture`gensym30))))](internalTmpFuture`gensym32))

Here is the problem. The previous implementation split the state on the case that there is a yield in the dest type of the cast. I don't think it's reasonable. It downgrades the performance of chained await calls and happended to work before since it execute the code without the state transition assignment.

It was translated into

state 3:
read(cast[...](...))
state 4:
:envP.`:state` = 5

But it is supposed to have a state assignement in the state 3 part

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 9e1d0d1

Hint: mm: orc; opt: speed; options: -d:release
178495 lines; 8.481s; 752.418MiB peakmem