improvements for destructors

tests/run/tdestructor.nim

@@ -0,0 +1,26 @@
+discard """
+  output: '''some text
+Destructor called!'''
+"""
+
+type
+  TMyObj = object
+    x, y: int
+    p: pointer
+    
+proc destruct(o: var TMyObj) {.destructor.} =
+  if o.p != nil: dealloc o.p
+  echo "Destructor called!"
+  
+proc open: TMyObj =
+  # allow for superfluous ()
+  result = (TMyObj(x: 1, y: 2, p: alloc(3)))
+
+
+proc `$`(x: TMyObj): string = $x.y
+
+proc main() =
+  var x = open()
+  echo "some text"
+
+main()