UncheckedSendable: AsyncSequence

Currently, a non-sendable sequence cannot be erased to `AsyncStream`
using this library's initializers even if one takes care to traffic it
through via `nonisolated(unsafe)`:

```swift
nonisolated(unsafe) let nonSendable = nonSendable
AsyncStream(nonSendable)  // 🛑
```

This conditional conformance acts as a workaround:

```swift
AsyncStream(UncheckedSendable(nonSendable))
```

Ideally folks can stop using our concrete async sequence eraser in favor
of `any AsyncSequence<Element, Failure>`, but this requires a minimum
deployment target of iOS 18, so it won't be an option for many people
for some time.

Author: stephencelis

Sources/ConcurrencyExtras/UncheckedSendable.swift

@@ -55,6 +55,12 @@ public struct UncheckedSendable<Value>: @unchecked Sendable {
   }
 }
 
+extension UncheckedSendable: AsyncSequence where Value: AsyncSequence {
+  public func makeAsyncIterator() -> Value.AsyncIterator {
+    value.makeAsyncIterator()
+  }
+}
+
 #if swift(>=5.10)
   @available(iOS, deprecated: 9999, message: "Use 'nonisolated(unsafe) let', instead.")@available(
     macOS, deprecated: 9999, message: "Use 'nonisolated(unsafe) let', instead."