implemented generic converters

tests/compile/tsecondarrayproperty.nim

@@ -0,0 +1,28 @@
+
+type
+  TFoo = object
+    data: array[0..100, int]
+  TSecond = distinct TFoo
+
+proc `[]` (self: var TFoo, x: int): var int =
+  return self.data[x]
+
+proc `[]=` (self: var TFoo, x, y: int) =
+  # only `[]` returning a 'var T' seems to not work for now :-/
+  self.data[x] = y
+
+proc second(self: var TFoo): var TSecond =
+  return TSecond(self)
+
+proc `[]`(self: var TSecond, x: int): var int =  
+  return TFoo(self).data[2*x]
+
+var f: TFoo
+
+for i in 0..f.data.high: f[i] = 2 * i
+
+echo f.second[1]
+
+#echo `second[]`(f,1)
+# this is the only way I could use it, but not what I expected
+