since now takes an optional patch, eg: since: (1, 3, 1) (#14124)

add tests for tinclrtl

lib/system.nim

@@ -2055,7 +2055,11 @@ export dollars
 
 const
   NimMajor* {.intdefine.}: int = 1
-    ## is the major number of Nim's version.
+    ## is the major number of Nim's version. Example:
+    ##
+    ## .. code-block:: Nim
+    ##   when (NimMajor, NimMinor, NimPatch) >=  (1, 3, 1): discard
+    # See also private symbol `since: (1, 3)` reserved for stdlib
 
   NimMinor* {.intdefine.}: int = 3
     ## is the minor number of Nim's version.

lib/system/inclrtl.nim

@@ -49,8 +49,20 @@ when defined(nimlocks):
 else:
   {.pragma: benign, gcsafe.}
 
+template isSince(version: (int, int)): bool =
+  (NimMajor, NimMinor) >= version
+template isSince(version: (int, int, int)): bool =
+  (NimMajor, NimMinor, NimPatch) >= version
+
 template since(version, body: untyped) {.dirty, used.} =
-  ## limitation: can't be used to annotate a template (eg typetraits.get), would
+  ## Usage:
+  ##
+  ## .. code-block:: Nim
+  ##   proc fun*() {since: (1, 3).}
+  ##   proc fun*() {since: (1, 3, 1).}
+  ##
+  ## Limitation: can't be used to annotate a template (eg typetraits.get), would
   ## error: cannot attach a custom pragma.
-  when (NimMajor, NimMinor) >= version:
+  # `dirty` needed because `NimMajor` may not yet be defined in caller.
+  when isSince(version):
     body