closures have an object type field so that we can perform correct deepCopy() for the BoehmGC target

tests/closure/tboehmdeepcopy.nim

@@ -0,0 +1,17 @@
+discard """
+  cmd: "nim c --gc:boehm $options $file"
+  output: '''meep'''
+"""
+
+proc callit(it: proc ()) =
+  it()
+
+proc main =
+  var outer = "meep"
+  proc x =
+    echo outer
+  var y: proc()
+  deepCopy(y, x)
+  callit(y)
+
+main()