asyncChannel: fixes potential crashes when no more awaiting (apple#143)

This commit fixes crashes where the state is "awaiting" with no more awaiting clients.
The state is now set to .idle to reflect the reality.
# Conflicts:
#	Sources/AsyncAlgorithms/AsyncChannel.swift

Author: twittemb

Sources/AsyncAlgorithms/AsyncChannel.swift

@@ -70,7 +70,11 @@ public final class AsyncChannel<Element: Sendable>: AsyncSequence, Sendable {
       switch self {
       case .awaiting(var awaiting):
         let continuation = awaiting.remove(Awaiting(placeholder: generation))?.continuation
-        self = .awaiting(awaiting)
+        if awaiting.isEmpty {
+          self = .idle
+        } else {
+          self = .awaiting(awaiting)
+        }
         return continuation
       default:
         return nil
@@ -117,8 +121,15 @@ public final class AsyncChannel<Element: Sendable>: AsyncSequence, Sendable {
           }
           return UnsafeResumption(continuation: send, success: continuation)
         case .awaiting(var nexts):
-          nexts.insert(Awaiting(generation: generation, continuation: continuation))
-          state.emission = .awaiting(nexts)
+          if nexts.update(with: Awaiting(generation: generation, continuation: continuation)) != nil {
+            nexts.remove(Awaiting(placeholder: generation))
+            cancelled = true
+          }
+          if nexts.isEmpty {
+            state.emission = .idle
+          } else {
+            state.emission = .awaiting(nexts)
+          }
           return nil
         }
       }?.resume()