switch to coroutine execution model

src/PSOProtocol.hh

@@ -1,6 +1,5 @@
 #pragma once
 
-#include <event2/bufferevent.h>
 #include <inttypes.h>
 
 #include <functional>
@@ -56,13 +55,13 @@ RetT& check_size_generic(
     size_t min_size,
     size_t max_size) {
   if (size < min_size) {
-    throw std::runtime_error(phosg::string_printf(
-        "command too small (expected at least 0x%zX bytes, received 0x%zX bytes)",
+    throw std::runtime_error(std::format(
+        "command too small (expected at least 0x{:X} bytes, received 0x{:X} bytes)",
         min_size, size));
   }
   if (size > max_size) {
-    throw std::runtime_error(phosg::string_printf(
-        "command too large (expected at most 0x%zX bytes, received 0x%zX bytes)",
+    throw std::runtime_error(std::format(
+        "command too large (expected at most 0x{:X} bytes, received 0x{:X} bytes)",
         max_size, size));
   }
   return *reinterpret_cast<RetT*>(data);