Remove AsyncIterator: Sendable requirement from merge

# Motivation
Currently a lot of the operator implementations in here that consume other `AsyncSequence`s require the `AsyncIterator` to be `Sendable`. This is mostly due to the fact that we are calling `makeAsyncIterator` on the upstream `AsyncSequence` and then pass that iterator around to various newly spawned `Task`s. This has two downsides:
1. It only allows users to use operators like `merge` if their `AsyncSequence.AsyncIterator` is `Sendable`
2. In merge we are creating new `Task`s for every new demand. Creating `Task`s is not cheap.

My main goal of this PR was to remove the `Sendable` constraint from `merge`.

# Modification
This PR overhauls the complete inner workings of the `AsyncMerge2Sequence`. It does a couple of things:
1. The main change is that instead of creating new `Task`s for every demand, we are creating one `Task` when the `AsyncIterator` is created. This task has as child task for every upstream sequence.
2. When calling `next` we are signalling the child tasks to demand from the upstream
3. A new state machine that is synchronizing the various concurrent operations that can happen
4. Handling cancellation since we are creating a bunch of continuations.

# Result
In the end, this PR swaps the implementation of `AsyncMerge2Sequence` and drops the `Sendable` constraint and passes all tests. Furthermore, on my local performance testing I saw up 50% speed increase in throughput.

# Open points
1. I need to make this sequence re-throwing but before going down that rabbit whole I wanna get buy-in on the implementation.
2. We should discuss and document if `merge` and other operators are hot or cold, i.e. if they only request if they got downstream demand
3. I need to switch `AsyncMerge3Sequence` over to the same iplementation

Author: FranzBusch

Package.swift

@@ -16,9 +16,16 @@ let package = Package(
     .library(name: "_CAsyncSequenceValidationSupport", type: .static, targets: ["AsyncSequenceValidation"]),
     .library(name: "AsyncAlgorithms_XCTest", targets: ["AsyncAlgorithms_XCTest"]),
   ],
-  dependencies: [],
+  dependencies: [
+    .package(url: "https://github.com/apple/swift-collections.git", from: "1.0.2"),
+  ],
   targets: [
-    .target(name: "AsyncAlgorithms"),
+    .target(
+        name: "AsyncAlgorithms",
+        dependencies: [
+            .product(name: "DequeModule", package: "swift-collections")
+        ]
+    ),
     .target(
       name: "AsyncSequenceValidation",
       dependencies: ["_CAsyncSequenceValidationSupport", "AsyncAlgorithms"]),