bring back typetraits.$ for backward compatibility

Some packages are still using ``from typetraits import `$` ``
which is now available from system (no need to import it).

lib/pure/typetraits.nim

@@ -10,6 +10,8 @@
 ## This module defines compile-time reflection procs for
 ## working with types.
 
+export system.`$` # for backward compatibility
+
 
 proc name*(t: typedesc): string {.magic: "TypeTrait".}
   ## Returns the name of the given type.