Modernize: Use range-based for loops for readability (#762)

Also run clang-format on these files as requested

src/exp.cpp

@@ -12,8 +12,7 @@ namespace YAML {
 namespace Exp {
 unsigned ParseHex(const std::string& str, const Mark& mark) {
   unsigned value = 0;
-  for (std::size_t i = 0; i < str.size(); i++) {
-    char ch = str[i];
+  for (char ch : str) {
     int digit = 0;
     if ('a' <= ch && ch <= 'f')
       digit = ch - 'a' + 10;
@@ -132,5 +131,5 @@ std::string Escape(Stream& in) {
   std::stringstream msg;
   throw ParserException(in.mark(), std::string(ErrorMsg::INVALID_ESCAPE) + ch);
 }
-}
-}
+}  // namespace Exp
+}  // namespace YAML