Access implicit result trough envP in closures (#8471)

Reuse the existing machinery, most of the changes are only needed to
handle the `result = result` node in nkReturnStmt produced by the
closure iterator transform.

Fixes #338

tests/iter/t338.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''0
+1
+2
+3
+4
+'''
+"""
+
+proc moo(): iterator (): int =
+  iterator fooGen: int {.closure.} =
+    while true:
+      yield result
+      result.inc
+  return fooGen
+
+var foo = moo()
+
+for i in 0 .. 4:
+  echo foo()