Count entries with count() rather than equal_range plus distance (#2525)

Seven accessors each spelled the same two lines:

    auto r = x.equal_range(key);
    return static_cast<size_t>(std::distance(r.first, r.second));

The containers behind them all became detail::insertion_ordered_multimap
along the way, so count() is available and says what these functions
mean. The work is the same either way: equal_range() scans to the first
match and distance() then walks the restricted iterator to the end,
comparing keys at each step, which comes to one pass over the entries,
and count() is one pass too. This is a readability change, not a faster
one.

Covers get_header_value_count, Request and Response
get_trailer_value_count, Request::get_param_value_count,
MultipartFormData get_field_count and get_file_count, and
Result::get_request_header_value_count. The distance() call in
get_param_values() stays, since it sizes a reserve() and needs the range
anyway.

httplib.h

@@ -7600,8 +7600,7 @@ inline const char *get_header_value(const Headers &headers,
 
 inline size_t get_header_value_count(const Headers &headers,
                                      const std::string &key) {
-  auto r = headers.equal_range(key);
-  return static_cast<size_t>(std::distance(r.first, r.second));
+  return headers.count(key);
 }
 
 template <typename Map>
@@ -10566,8 +10565,7 @@ inline std::string Request::get_trailer_value(const std::string &key,
 }
 
 inline size_t Request::get_trailer_value_count(const std::string &key) const {
-  auto r = trailers.equal_range(key);
-  return static_cast<size_t>(std::distance(r.first, r.second));
+  return trailers.count(key);
 }
 
 inline bool Request::has_param(const std::string &key) const {
@@ -10591,8 +10589,7 @@ Request::get_param_values(const std::string &key) const {
 }
 
 inline size_t Request::get_param_value_count(const std::string &key) const {
-  auto r = params.equal_range(key);
-  return static_cast<size_t>(std::distance(r.first, r.second));
+  return params.count(key);
 }
 
 inline bool Request::is_multipart_form_data() const {
@@ -10625,8 +10622,7 @@ inline bool MultipartFormData::has_field(const std::string &key) const {
 }
 
 inline size_t MultipartFormData::get_field_count(const std::string &key) const {
-  auto r = fields.equal_range(key);
-  return static_cast<size_t>(std::distance(r.first, r.second));
+  return fields.count(key);
 }
 
 inline FormData MultipartFormData::get_file(const std::string &key,
@@ -10649,8 +10645,7 @@ inline bool MultipartFormData::has_file(const std::string &key) const {
 }
 
 inline size_t MultipartFormData::get_file_count(const std::string &key) const {
-  auto r = files.equal_range(key);
-  return static_cast<size_t>(std::distance(r.first, r.second));
+  return files.count(key);
 }
 
 // Multipart FormData writer implementation
@@ -10729,8 +10724,7 @@ inline std::string Response::get_trailer_value(const std::string &key,
 }
 
 inline size_t Response::get_trailer_value_count(const std::string &key) const {
-  auto r = trailers.equal_range(key);
-  return static_cast<size_t>(std::distance(r.first, r.second));
+  return trailers.count(key);
 }
 
 inline void Response::set_redirect(const std::string &url, int stat) {
@@ -10826,8 +10820,7 @@ inline std::string Result::get_request_header_value(const std::string &key,
 
 inline size_t
 Result::get_request_header_value_count(const std::string &key) const {
-  auto r = request_headers_.equal_range(key);
-  return static_cast<size_t>(std::distance(r.first, r.second));
+  return request_headers_.count(key);
 }
 
 // Stream implementation