fix varargs forwarding for templates; fixes #5455 (#5505)

* fix varargs forwarding for templates; fixes #5455
* document the macros' varargs change in the news for 0.16.2

tests/overload/tparam_forwarding.nim

@@ -0,0 +1,37 @@
+discard """
+output: '''baz
+10
+100
+1000
+a
+b
+c
+'''
+"""
+
+type
+  Foo = object
+    x: int
+
+proc stringVarargs*(strings: varargs[string, `$`]): void =
+  for s in strings: echo s
+
+proc fooVarargs*(foos: varargs[Foo]) =
+  for f in foos: echo f.x
+
+template templateForwarding*(callable: untyped,
+                             condition: bool,
+                             forwarded: varargs[untyped]): untyped =
+  if condition:
+    callable(forwarded)
+
+proc procForwarding(args: varargs[string]) =
+  stringVarargs(args)
+
+templateForwarding stringVarargs, 17 + 4 < 21, "foo", "bar", 100
+templateForwarding stringVarargs, 10 < 21, "baz"
+
+templateForwarding fooVarargs, "test".len > 3, Foo(x: 10), Foo(x: 100), Foo(x: 1000)
+
+procForwarding "a", "b", "c"
+