Allow constructon of json_stringt from irep_idt.

Allow constructon of `jsont`/`json_stringt` from
`std::string`/`irep_idt`/string literals. This allows for a
reduction in the amount of boilerplate code. For example instead of
writing
```
result["file"] = json_stringt(id2string(location.get_file()));
```
we could write
```
result["file"] = json_stringt(location.get_file());
```
The constructor from string literal is required when constructors from
both `std::string` and `irep_idt` are provided. Otherwise the compiler
does not know whether to implicitly convert string literals into
a `std:string` or a `irep_idt`.

Author: thomasspriggs

src/util/json.h

@@ -15,6 +15,8 @@ Author: Daniel Kroening, [email protected]
 #include <iosfwd>
 #include <string>
 
+#include "irep.h"
+
 class json_objectt;
 class json_arrayt;
 
@@ -182,8 +184,20 @@ class json_arrayt:public jsont
 class json_stringt:public jsont
 {
 public:
-  explicit json_stringt(const std::string &_value):
-    jsont(kindt::J_STRING, _value)
+  explicit json_stringt(std::string _value):
+    jsont(kindt::J_STRING, std::move(_value))
+  {
+  }
+
+#ifndef USE_STD_STRING
+  explicit json_stringt(const irep_idt &_value)
+    : jsont(kindt::J_STRING, id2string(_value))
+  {
+  }
+#endif
+
+  /// Constructon from string literal.
+  explicit json_stringt(const char *_value) : jsont(kindt::J_STRING, _value)
   {
   }
 };