add begin() and end() to dstring container

Author: Daniel Kroening

src/solvers/smt2/smt2_format.cpp

@@ -59,11 +59,9 @@ std::ostream &smt2_format_rec(std::ostream &out, const exprt &expr)
     }
     else if(expr_type.id() == ID_string)
     {
-      const auto &value_string = id2string(value);
-
       out << '"';
 
-      for(const auto &c : value_string)
+      for(const auto &c : value)
       {
         // " is the only escape sequence
         if(c == '"')

src/util/dstring.h

@@ -151,6 +151,17 @@ class dstringt final
     return no;
   }
 
+  // iterators for the underlying string
+  std::string::const_iterator begin() const
+  {
+    return as_string().begin();
+  }
+
+  std::string::const_iterator end() const
+  {
+    return as_string().end();
+  }
+
 private:
   #ifdef __GNUC__
   constexpr