[coro_io] add random engine

Author: poor-circle

include/ylt/util/random.hpp

@@ -0,0 +1,8 @@
+#include <random>
+namespace ylt::util {
+template <typename engine_type = std::default_random_engine>
+inline engine_type& random_engine() {
+  static thread_local std::default_random_engine e(std::random_device{}());
+  return e;
+}
+}  // namespace ylt::util