[WIP] Akka.IO / Akka.Streams: `ByteString` rewrite

[Aaronontheweb]

Changes

A leftover milestone from v1.5 - we are dragging ByteString kicking and screaming into the .NET Core era with major refactoring, namely replacing all of its internals ReadOnlyMemory<byte> usage.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• [ ] This change follows the Akka.NET API Compatibility Guidelines.
• [ ] This change follows the Akka.NET Wire Compatibility Guidelines.
• [ ] I have reviewed my own pull request.
• [ ] Design discussion issue #
• [ ] Changes in public API reviewed, if any.
• [ ] I have added website documentation for this feature.

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[Aaronontheweb]

Tests are not currently compiling - working on that.

[Aaronontheweb]

Let's see how accurately ChatGPT was able to describe the current FSM behavior of Akka.IO's TcpConnection actor, one of the areas I'm looking to refactor:

stateDiagram-v2
    [*] --> WaitingForRegistration: Connection established
    WaitingForRegistration --> Connected: Register
    WaitingForRegistration --> Closing: CloseCommand
    WaitingForRegistration --> WaitingForRegistration: ResumeReading / SuspendReading
    Connected --> ClosingWithPendingWrite: CloseCommand with pending write
    Connected --> PeerSentEOF: PeerClosed (keep open)
    Connected --> Closing: CloseCommand
    Connected --> Connected: WriteCommand / SocketSent / SuspendReading / ResumeReading
    PeerSentEOF --> Closing: CloseCommand
    ClosingWithPendingWrite --> Closing: Write completed
    Closing --> Closed: Connection closed
    Closed --> [*]

[Aaronontheweb]

Honestly, not a bad job ChatGPT ^

[Aaronontheweb]

Looks like SocketAsyncEventArgs is still the way to go for high performance sockets in .NET: https://twitter.com/_MihaZupan/status/1774911865019113974

[Aaronontheweb]

Gonna need to redo this - stuff like the framing code is phenomenally messy and difficult to reason about. That needs to be refactored and simplified first.