message: Add Co::exists.
diff: @@ -60,12 +60,16 @@ ZEND_BEGIN_ARG_INFO_EX(arginfo_swoole_coroutine_create, 0, 0, 1)
  ZEND_ARG_VARIADIC_INFO(0, params)
  ZEND_END_ARG_INFO()
  -ZEND_BEGIN_ARG_INFO_EX(arginfo_swoole_coroutine_exec, 0, 0, 1)
  - ZEND_ARG_INFO(0, command)
  +ZEND_BEGIN_ARG_INFO_EX(arginfo_swoole_coroutine_resume, 0, 0, 1)
  + ZEND_ARG_INFO(0, cid)
  ZEND_END_ARG_INFO()
  -ZEND_BEGIN_ARG_INFO_EX(arginfo_swoole_coroutine_resume, 0, 0, 1)
  - ZEND_ARG_INFO(0, uid)
  +ZEND_BEGIN_ARG_INFO_EX(arginfo_swoole_coroutine_exists, 0, 0, 1)
  + ZEND_ARG_INFO(0, cid)
  +ZEND_END_ARG_INFO()
  +
  +ZEND_BEGIN_ARG_INFO_EX(arginfo_swoole_coroutine_exec, 0, 0, 1)
  + ZEND_ARG_INFO(0, command)
  ZEND_END_ARG_INFO()
  ZEND_BEGIN_ARG_INFO_EX(arginfo_swoole_coroutine_sleep, 0, 0, 1)
  @@ -126,6 +130,7 @@ ZEND_BEGIN_ARG_INFO_EX(arginfo_swoole_coroutine_getBackTrace, 0, 0, 1)
  ZEND_END_ARG_INFO()
  static PHP_METHOD(swoole_coroutine_util, set);
  +static PHP_METHOD(swoole_coroutine_util, exists);
  static PHP_METHOD(swoole_coroutine_util, yield);
  static PHP_METHOD(swoole_coroutine_util, resume);
  static PHP_METHOD(swoole_coroutine_util, stats);
  @@ -177,6 +182,7 @@ static const zend_function_entry swoole_coroutine_util_methods[] =
  ZEND_FENTRY(gethostbyname, ZEND_FN(swoole_coroutine_gethostbyname), arginfo_swoole_coroutine_gethostbyname, ZEND_ACC_PUBLIC | ZEND_ACC_STATIC)
  ZEND_FENTRY(defer, ZEND_FN(swoole_coroutine_defer), arginfo_swoole_coroutine_defer, ZEND_ACC_PUBLIC | ZEND_ACC_STATIC)
  PHP_ME(swoole_coroutine_util, set, arginfo_swoole_coroutine_set, ZEND_ACC_PUBLIC | ZEND_ACC_STATIC)
  + PHP_ME(swoole_coroutine_util, exists, arginfo_swoole_coroutine_exists, ZEND_ACC_PUBLIC | ZEND_ACC_STATIC)
  PHP_ME(swoole_coroutine_util, yield, arginfo_swoole_coroutine_void, ZEND_ACC_PUBLIC | ZEND_ACC_STATIC)
  PHP_MALIAS(swoole_coroutine_util, suspend, yield, arginfo_swoole_coroutine_void, ZEND_ACC_PUBLIC | ZEND_ACC_STATIC)
  PHP_ME(swoole_coroutine_util, resume, arginfo_swoole_coroutine_resume, ZEND_ACC_PUBLIC | ZEND_ACC_STATIC)
  @@ -333,9 +339,11 @@ static PHP_METHOD(swoole_exit_exception, getStatus)
  RETURN_ZVAL(sw_zend_read_property(Z_OBJCE_P(getThis()), getThis(), ZEND_STRL("status"), 1), 1, 0);
  }
  -/*
  - * suspend current coroutine
  - */
  +static PHP_METHOD(swoole_coroutine_util, exists)
  +{
  + RETURN_BOOL(Coroutine::get_current() != nullptr);
  +}
  +
  static PHP_METHOD(swoole_coroutine_util, yield)
  {
  Coroutine* co = Coroutine::get_current();
  