adds default template for set[T] (#1205)

It is required to compile nifreader with Nimony.

Author: demotomohiro

lib/std/system/defaults.nim

@@ -28,3 +28,5 @@ proc default*[T: tuple](x: typedesc[T]): T {.magic: DefaultTup.}
 proc default*[I: Ordinal; T: HasDefault](x: typedesc[array[I, T]]): array[I, T] {.inline, noinit, nodestroy.} =
   for i in low(array[I, T]) .. high(array[I, T]):
     result[i] = default(T)
+
+template default*[T](x: typedesc[set[T]]): set[T] = {}