improvements for destructors

tests/reject/tdestructor.nim

@@ -0,0 +1,21 @@
+discard """
+  line: 20
+  errormsg: " usage of a type with a destructor in a non destructible context"
+"""
+
+type  
+  TMyObj = object
+    x, y: int
+    p: pointer
+    
+proc destruct(o: var TMyObj) {.destructor.} =
+  if o.p != nil: dealloc o.p
+  
+proc open: TMyObj =
+  result = TMyObj(x: 1, y: 2, p: alloc(3))
+
+
+proc `$`(x: TMyObj): string = $x.y
+
+echo open()
+