Supporting ? as a string presentation type (#2674)

* Supporting ? as a string presentation type.

* Supporting ? as a char presentation type.

* Adding iterator_category to counting_iterator.

include/fmt/compile.h

@@ -13,45 +13,6 @@
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
-// An output iterator that counts the number of objects written to it and
-// discards them.
-class counting_iterator {
- private:
-  size_t count_;
-
- public:
-  using iterator_category = std::output_iterator_tag;
-  using difference_type = std::ptrdiff_t;
-  using pointer = void;
-  using reference = void;
-  using _Unchecked_type = counting_iterator;  // Mark iterator as checked.
-
-  struct value_type {
-    template <typename T> void operator=(const T&) {}
-  };
-
-  counting_iterator() : count_(0) {}
-
-  size_t count() const { return count_; }
-
-  counting_iterator& operator++() {
-    ++count_;
-    return *this;
-  }
-  counting_iterator operator++(int) {
-    auto it = *this;
-    ++*this;
-    return it;
-  }
-
-  friend counting_iterator operator+(counting_iterator it, difference_type n) {
-    it.count_ += static_cast<size_t>(n);
-    return it;
-  }
-
-  value_type operator*() const { return {}; }
-};
-
 template <typename Char, typename InputIt>
 inline counting_iterator copy_str(InputIt begin, InputIt end,
                                   counting_iterator it) {