Add io_uring integration for Linux performance optimization

[kajukitli]

Adds io_uring support for Linux performance optimization.

Performance

WebSocket benchmark results:

• tokio: ~4ms for 100 echo messages
• io_uring: ~3ms for 100 echo messages
• 20% performance improvement

Implementation

• io-uring feature flag with conditional compilation
• Drop-in replacement for tokio::net types
• Task-based AsyncRead/AsyncWrite adapter for io_uring compatibility
• Full WebSocket protocol support through existing APIs

Usage

// Enable with feature flag
use fastwebsockets::uring::{TcpStream, TcpListener, start};

start(async {
    let listener = TcpListener::bind(addr)?;
    let (stream, _) = listener.accept().await?;
    let mut ws = WebSocket::after_handshake(stream, Role::Server);
    // existing APIs work unchanged
});

Testing

cargo test --features io-uring
cargo run --example websocket_bench --release --features io-uring
cargo run --example simple_bench --release --features io-uring

Requirements

Linux 5.11+, x86_64/aarch64. Optional dependency on tokio-uring = "0.5.0".

Without the feature flag, uses standard tokio with zero overhead.