untangle includes

- order lexicographically
- include all used headers directly instead of implicit includes
- remove unused header
- move regex include into compile unit

src/cxxopts.cpp

@@ -24,6 +24,8 @@ THE SOFTWARE.
 
 #include "cxxopts.hpp"
 
+#include <regex>
+
 #define OPTION_LONGEST 30
 #define OPTION_DESC_GAP 2
 

src/cxxopts.hpp

@@ -22,13 +22,13 @@ THE SOFTWARE.
 
 */
 
-#include <regex>
-#include <set>
-#include <map>
 #include <exception>
-#include <sstream>
-
 #include <iostream>
+#include <map>
+#include <memory>
+#include <sstream>
+#include <string>
+#include <vector>
 
 namespace cxxopts
 {