v0.15.0

• Overhaul of the epoll and kqueue backends, to make them comparable to the performance of
  the io_uring backend. When migrating from libxev to our own event loop, I decided to use
  a similar approach for both backends, which really goes against the nature of these APIs.
  With this new rewrite, both backends keep fds registered in the kernel, so readiness is
  always available. This results in far fewer syscalls, and overall better performance.
  One side effect is that now tasks that were running on executor A can be moved to
  executor B, if the event loop B is where the fd is registered.

• Improved performance of net.Stream.Writer.sendFile on all platforms. There is now
  a native zero-copy implementation for Windows using TransmitFile, and the generic
  fallback now uses the entire reader/writer buffers, so it's always faster than the
  read/write loop fallback implemented in std.Io.Writer.

• Added File.stdReader/File.stdWriter to wrap a zio-opened file as the concrete
  std.Io.File.Reader/std.Io.File.Writer types, so it works with std.Io APIs that
  require them (like std.Io.Writer.sendFileAll).

• Implemented wall-clock timers, so you can now sleep/timeout using the real-time clock and be
  woken up exactly on time, even if the clock is adjusted. This is natively supported on Linux,
  but needs more careful coordination on other platforms.

• Added support for all clocks that std.Io supports (real, boot, awake, and the
  cpu_process/cpu_thread CPU-time clocks), as well as querying their resolution.

• Changed how stdin/stdout/stderr are handled on Windows, to make sure we can work
  with these without blocking the event loop, since they are not open as OVERLAPPED handles.

• Changed the io_uring backend from futex-based wake ups to eventfd, which works much
  more reliably. The previous futex approach introduced wake up latency that I could not explain.

• Error code ETIMEDOUT is now mapped to error.ConnectionTimedOut for send/recv operations.
  We are not using kernel-level socket timeouts, but it seems that these error codes can still happen.

• New TaskLocal API for storing custom task-local data.

• Added custom random and randomSecure APIs for generating random numbers,
  to reduce dependency on std.Io.Threaded.

• Fixed handling of Unix socket addresses containing null bytes.

• Fixed race in cross-thread handling of AcceptEx calls on Windows.

• Fixed shutdown sequence to properly stop the thread pool before closing the event loop.

• Fixed memory leak that happens after spawning blocking tasks on the thread pool.