stdlib: use system.default if it exists

lib/pure/collections/sets.nim

@@ -74,7 +74,8 @@ type
 
 const growthFactor = 2
 
-template default[T](t: typedesc[T]): T =
+when not defined(nimHasDefault):
+  template default[T](t: typedesc[T]): T =
     ## Used by clear methods to get a default value.
     var v: T
     v
@@ -162,10 +163,6 @@ template doWhile(a, b) =
     b
     if not a: break
 
-template default[T](t: typedesc[T]): T =
-  var v: T
-  v
-
 proc exclImpl[A](s: var HashSet[A], key: A) : bool {. inline .} =
   assert s.isValid, "The set needs to be initialized."
   var hc: Hash

lib/pure/collections/tableimpl.nim

@@ -116,7 +116,8 @@ template hasKeyOrPutImpl(enlarge) {.dirty.} =
     maybeRehashPutImpl(enlarge)
   else: result = true
 
-template default[T](t: typedesc[T]): T =
+when not defined(nimHasDefault):
+  template default[T](t: typedesc[T]): T =
     var v: T
     v