Add async/await support with tokio and async-std

[charmitro]

Add async/await API for io_uring operations with support for both tokio and async-std runtimes. This enables idiomatic Rust async programming patterns with io_uring's high-performance I/O.

Features:

• AsyncIoUring wrapper for async operations
• tokio integration using AsyncFd for event loop integration
• async-std integration using spawn_blocking
• Conditional compilation with async-tokio and async-async-std features

Implementation details:

• tokio: Uses AsyncFd to integrate io_uring fd with tokio's event loop, achieving 1.9M ops/sec in poll benchmarks
• async-std: Uses spawn_blocking for background execution, achieving 234K ops/sec
• Both runtimes support all io_uring operations through the PrepareOp trait
• Thread-safe design using Arc<Mutex<>> for shared ring access
• Proper waker management for concurrent operation handling

Examples added:

• examples/async_nop_tokio.rs: Basic async NOP with tokio
• examples/async_nop_async_std.rs: Basic async NOP with async-std
• examples/async_poll_bench.rs: Polling benchmark with tokio
• examples/async_poll_bench_async_std.rs: Polling benchmark with async-std