multisync now allows tuples in return type (#21074)

* Add test case

* Use .toStrLit() on param node first

This means that more complex types are fully rendered

tests/async/t20111.nim

@@ -0,0 +1,19 @@
+discard """
+  action: "run"
+"""
+import asyncdispatch
+type
+    Sync = object
+    Async = object
+    SyncRes = (Sync, string)
+    AsyncRes = (Async, string)
+
+proc foo(val: Sync | Async): Future[(Async, string) | (Sync, string)] {.multisync.} =
+    return (val, "hello")
+
+let
+  myAsync = Async()
+  mySync = Sync()
+
+doAssert typeof(waitFor foo(myAsync)) is AsyncRes
+doAssert typeof(foo(mySync)) is SyncRes