message: Add bytes_available function
diff: @@ -207,6 +207,16 @@ impl VirtualFile for wasi_pipe_t {
  fn unlink(&mut self) -> Result<(), FsError> {
  Ok(())
  }
  + fn bytes_available(&self) -> Result<usize, FsError> {
  + Ok(self.bytes_available_read()?.unwrap_or(0usize)
  + + self.bytes_available_write()?.unwrap_or(0usize))
  + }
  + fn bytes_available_read(&self) -> Result<Option<usize>, FsError> {
  + Ok(None)
  + }
  + fn bytes_available_write(&self) -> Result<Option<usize>, FsError> {
  + Ok(None)
  + }
  }
  #[no_mangle]
  