code gen error when using sequtils.mapIt()
Example code to illustrate  (it fails for me on win7, don't know about other Linux/Mac)

```
import jester, asyncdispatch, htmlgen
import strutils, sequtils

var
  tseq = @[@["Hello","World"],@["bad","result"],@["Needs a","fix"]]

proc doTest(): string =
  var
    thed  = ""
    tbod = ""
  result = ""
  for rNr,r in pairs(tseq):       # for each row (a seq[string])
    var x: seq[string] = r
    if rNr == 0:                  # header row
      x = r.mapIt(th(it))  # THIS FAILS
      #x.applyIt(th(it))   # THIS WORKS
      thed = x.join(" ")  # str join wrapped th elements
    else:
      # add row wrapped in tr, with each element tt wrapped
      #x = r.mapIt(td(it))  # THIS FAILS
      x.applyIt(td(it))   # THIS WORKS
      tbod &= tr(x.join(" ")) 
  result = table(thead(thed), tbody(tbod))

proc test2(): string = "test data"

echo html(body(doTest()))

routes:
  get "/test2":
    resp html(body(test2()))    # works
  get "/test1":
    resp html(body(doTest()))   # doesn't work if contains mapIt()

runForever()
```

errors like:

```
nimcache\compiler_bug1.c: In function 'matchiter_250403':
nimcache\compiler_bug1.c:949:26: error: request for member 'ClEnv' in something not a structure or union
     LOC26 = dotest_250060.ClEnv? dotest_250060.ClPrc(dotest_250060.ClEnv):((TMP1315)(dotest_250060.ClPrc))();
                          ^
nimcache\compiler_bug1.c:949:47: error: request for member 'ClPrc' in something not a structure or union
     LOC26 = dotest_250060.ClEnv? dotest_250060.ClPrc(dotest_250060.ClEnv):((TMP1315)(dotest_250060.ClPrc))();
                                               ^
nimcache\compiler_bug1.c:949:67: error: request for member 'ClEnv' in something not a structure or union
     LOC26 = dotest_250060.ClEnv? dotest_250060.ClPrc(dotest_250060.ClEnv):((TMP1315)(dotest_250060.ClPrc))();
                                                                   ^
nimcache\compiler_bug1.c:949:99: error: request for member 'ClPrc' in something not a structure or union
     LOC26 = dotest_250060.ClEnv? dotest_250060.ClPrc(dotest_250060.ClEnv):((TMP1315)(dotest_250060.ClPrc))();

```
