the is operator now works with type classes and type variables

bugfixes:
the DLL tests were failing on Mac OS X, due to an incorrect DynlibFormat

tests/reject/tcaseexpr1.nim

@@ -1,11 +1,11 @@
 discard """
   file: "tcaseexpr1.nim"
 
-  line: 23
-  errormsg: "not all cases are covered"
-
   line: 29
   errormsg: "type mismatch: got (string) but expected 'int'"
+
+  line: 23
+  errormsg: "not all cases are covered"
 """
 
 type

tests/reject/tenummix.nim

@@ -1,6 +1,6 @@
 discard """
   file: "system.nim"
-  line: 643
+  line: 649
   errormsg: "type mismatch"
 """