next steps for closure iterators

tests/objects/tobjcov.nim

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+# Covariance is not type safe:
+
+type
+  TA = object of TObject
+    a: int
+  TB = object of TA
+    b: array[0..5000_000, int]
+    
+proc ap(x: var TA) = x.a = -1
+proc bp(x: var TB) = x.b[high(x.b)] = -1
+    
+# in Nimrod proc (x: TB) is compatible to proc (x: TA),
+# but this is not type safe:
+var f = cast[proc (x: var TA) {.nimcall.}](bp)
+var a: TA
+f(a) # bp expects a TB, but gets a TA
+