added 'since' template for further stdlib additions

(cherry picked from commit 372b01711e)

lib/system/inclrtl.nim

@@ -48,3 +48,7 @@ when defined(nimlocks):
   {.pragma: benign, gcsafe, locks: 0.}
 else:
   {.pragma: benign, gcsafe.}
+
+template since(version, body: untyped) {.dirty.} =
+  when version <= (NimMajor, NimMinor):
+    body