Iterators broken or maybe better docs needed on usage

[lilkeet]

I would think that this is how you'd use a contractual iterator:

import
  std / [sequtils],
  contracts

iterator myIter(x: openArray[int]): int {.contractual.} =
  require:
    x.len == 11
  ensure:
    true
  body:
    for i in x:
      yield i



for i in (0..10).toSeq.myIter:
  echo i

But the ast produced is illformed:

/path/to/test.nim(5, 43) template/generic instantiation of `contractual` from here
/path/to/.nimble/pkgs2/contracts-0.2.2-511e9dbea3f5cab286a34f1bbe760bdf96c7b250/contracts/contexts/handlers/conIter.nim(51, 10) Warning: Loop contracts are known to be buggy as for now, use with caution. [User]
/path/to/test.nim(5, 43) template/generic instantiation of `contractual` from here
/path/to/test.nim(10, 3) Error: illformed AST: block key_impl:
  for i in x:
    yielded = i
    yield i

If my current pr is responded to I'll dive in and try to fix this, then the documentation bug.