bugfix: threads should work again; fixes #220

tests/run/tmixin.nim

@@ -0,0 +1,27 @@
+discard """
+  output: "1\n2"
+"""
+
+type
+  TFoo1 = object of TObject
+    v: int
+  TFoo2 = object of TFoo1
+    v2: int
+
+proc test(f: TFoo1) =
+  echo "1"
+
+proc Foo[T](f: T) =
+  mixin test
+  test(f)
+
+var
+  a: TFoo1
+  b: TFoo2
+
+
+proc test(f: TFoo2) =
+  echo "2"
+
+Foo(a)
+Foo(b)