Add runOnce proc

This is useful for forcing progress when embedding Syndicate in
contexts that don't expose turn semantics.

src/syndicate/actors.nim

@@ -736,8 +736,24 @@ proc runPendingTurns* =
       terminateActor(turn, err)
       raise err
 
+proc runOnce*: bool {.discardable.} =
+  ## Run pending turns if there are any, otherwise
+  ## poll for external events and run any resulting turns.
+  ## Return true if any turns have been processed.
+  if turnQueue.len == 0:
+    when defined(solo5):
+      discard solo5_dispatcher.runOnce()
+    else:
+      var ready: seq[Continuation]
+      ioqueue.poll(ready)
+      while ready.len > 0:
+        discard trampoline:
+          ready.pop()
+  result = turnQueue.len > 0
+  runPendingTurns()
+
 proc run* =
-  ## Run actors to completion
+  ## Run actors to completion.
   when defined(solo5):
     while turnQueue.len > 0 or solo5_dispatcher.runOnce():
       runPendingTurns()

syndicate.nimble

@@ -1,6 +1,6 @@
 # Package
 
-version = "20240422"
+version = "20240424"
 author        = "Emery Hemingway"
 description   = "Syndicated actors for conversational concurrency"
 license       = "Unlicense"