introduce nkTupleConstr AST node for unary tuple construction; breaking change

tests/tuples/tanontuples.nim

@@ -1,7 +1,10 @@
 discard """
-  output: '''61, 125'''
+  output: '''61, 125
+(Field0: 0) (Field0: 13)'''
 """
 
+import macros
+
 proc `^` (a, b: int): int =
   result = 1
   for i in 1..b: result = result * a
@@ -12,3 +15,12 @@ var n = (56, 3)
 m = (n[0] + m[1], m[1] ^ n[1])
 
 echo m[0], ", ", m[1]
+
+# also test we can produce unary anon tuples in a macro:
+macro mm(): untyped =
+  result = newTree(nnkTupleConstr, newLit(13))
+
+proc nowTuple(): (int,) =
+  result = (0,)
+
+echo nowTuple(), " ", mm()