replace old problematic isNamedTuple implementation by TypeTrait isNa…

…medTuple

lib/system/dollars.nim

@@ -49,18 +49,7 @@ proc `$`*(t: typedesc): string {.magic: "TypeTrait".}
   ##   doAssert $(type("Foo")) == "string"
   ##   static: doAssert $(type(@['A', 'B'])) == "seq[char]"
 
-
-proc isNamedTuple(T: typedesc): bool =
-  # Taken from typetraits.
-  when T isnot tuple: result = false
-  else:
-    var t: T
-    for name, _ in t.fieldPairs:
-      when name == "Field0":
-        return compiles(t.Field0)
-      else:
-        return true
-    return false
+proc isNamedTuple(T: typedesc): bool {.magic: "TypeTrait".}
 
 proc `$`*[T: tuple|object](x: T): string =
   ## Generic ``$`` operator for tuples that is lifted from the components