Add cast operator from ranget to a container

Adding `.collect` to the end of a chain of range operations can add
clutter to the code, rather than improving readability, in places where
the type of container being constructed is apparent from the context.
This commit adds a cast operation in order to improve readability in
this scenario.

In addition to the operator, this commit includes a refactor of
`optionst::to_json` in order to demonstrate an example usage of the
proposed change.

Author: thomasspriggs

src/util/options.cpp

@@ -11,7 +11,9 @@ Author: Daniel Kroening, [email protected]
 
 #include "options.h"
 
+#include "constructor_of.h"
 #include "json.h"
+#include "range.h"
 #include "string2int.h"
 #include "xml.h"
 
@@ -90,14 +92,12 @@ const optionst::value_listt &optionst::get_list_option(
 /// Returns the options as JSON key value pairs
 json_objectt optionst::to_json() const
 {
-  json_objectt json_options;
-  for(const auto &option_pair : option_map)
-  {
-    json_arrayt &values = json_options[option_pair.first].make_array();
-    for(const auto &value : option_pair.second)
-      values.push_back(json_stringt(value));
-  }
-  return json_options;
+  return make_range(option_map)
+    .map([](const std::pair<std::string, value_listt> &option_pair) {
+      return std::pair<std::string, json_arrayt>{
+        option_pair.first,
+        make_range(option_pair.second).map(constructor_of<json_stringt>())};
+    });
 }
 
 /// Returns the options in XML format

src/util/range.h

@@ -359,6 +359,12 @@ struct ranget final
     return {begin(), end()};
   }
 
+  template <typename containert>
+  operator containert() const
+  {
+    return collect<containert>();
+  }
+
 private:
   iteratort begin_value;
   iteratort end_value;