Make formatted_size part of the core API

include/fmt/format.h

@@ -3586,15 +3586,6 @@ std::basic_string<Char> detail::vformat(
   return to_string(buffer);
 }
 
-/**
-  Returns the number of characters in the output of
-  ``format(format_str, args...)``.
- */
-template <typename... Args>
-inline size_t formatted_size(string_view format_str, const Args&... args) {
-  return format_to(detail::counting_iterator(), format_str, args...).count();
-}
-
 template <typename Char, FMT_ENABLE_IF(std::is_same<Char, wchar_t>::value)>
 void vprint(std::FILE* f, basic_string_view<Char> format_str,
             wformat_args args) {