Export compiled_string so that user can customize one

include/fmt/compile.h

@@ -15,6 +15,12 @@
 #include "format.h"
 
 FMT_BEGIN_NAMESPACE
+
+FMT_BEGIN_EXPORT
+// A compile-time string which is compiled into fast formatting code.
+class compiled_string {};
+FMT_END_EXPORT
+
 namespace detail {
 
 template <typename T, typename InputIt>
@@ -23,9 +29,6 @@ FMT_CONSTEXPR inline auto copy(InputIt begin, InputIt end, counting_iterator it)
   return it + (end - begin);
 }
 
-// A compile-time string which is compiled into fast formatting code.
-class compiled_string {};
-
 template <typename S>
 struct is_compiled_string : std::is_base_of<compiled_string, S> {};
 
@@ -42,7 +45,7 @@ struct is_compiled_string : std::is_base_of<compiled_string, S> {};
  */
 #if defined(__cpp_if_constexpr) && defined(__cpp_return_type_deduction)
 #  define FMT_COMPILE(s) \
-    FMT_STRING_IMPL(s, fmt::detail::compiled_string, explicit)
+    FMT_STRING_IMPL(s, fmt::compiled_string, explicit)
 #else
 #  define FMT_COMPILE(s) FMT_STRING(s)
 #endif