treewide: Switch to std::format

Reduce our dependence on fmtlib.

Change-Id: I1cc9b372aa366ae26d846470c8ab9d1d52e8db70
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/dl.cpp b/src/dl.cpp
index 050f25c..98ecae0 100644
--- a/src/dl.cpp
+++ b/src/dl.cpp
@@ -1,8 +1,9 @@
 #include <dlfcn.h>
-#include <fmt/format.h>
 
 #include <stdplus/dl.hpp>
 
+#include <format>
+
 namespace stdplus
 {
 
@@ -30,7 +31,7 @@
     void* ret = ::dlopen(file, flags);
     if (ret == nullptr)
     {
-        throw std::runtime_error(fmt::format(
+        throw std::runtime_error(std::format(
             "dlopen `{}`: {}", file ? file : "<nullptr>", dlerror()));
     }
     return ret;