add compat shim for std::to_underlying

llarp/util/underlying.hpp

@@ -0,0 +1,17 @@
+#pragma once
+#include <utility>
+
+namespace llarp
+{
+
+#ifdef __cpp_lib_to_underlying
+  using to_underlying = std::to_underlying;
+#else
+  template <typename T>
+  constexpr auto
+  to_underlying(T t)
+  {
+    return static_cast<std::underlying_type_t<T>>(t);
+  }
+#endif
+}  // namespace llarp