i2string -> std::to_string

i2string has gone from upstream cbmc as it is redundant; use
std::to_string instead.

Author: smowton

src/taint-analysis/taint_set.cpp

@@ -1,6 +1,5 @@
 #include "taint_set.h"
 #include <util/string2int.h>
-#include <util/i2string.h>
 #include <algorithm>
 #include <cassert>
 

src/taint-analysis/taint_summary_json.cpp

@@ -3,7 +3,6 @@
 #include "taint_summary.h"
 
 #include "util/json_irep.h"
-#include "util/i2string.h"
 #include "util/string2int.h"
 
 #include <algorithm>

src/taint-analysis/taint_variable.cpp

@@ -1,6 +1,5 @@
 #include "taint_variable.h"
 #include <util/string2int.h>
-#include <util/i2string.h>
 #include <cassert>
 
 
@@ -25,5 +24,5 @@ taint_variablet taint_variablet::from_json(
 jsont taint_variablet::to_json(
   const taint_tokent::named_tokenst &named_tokens) const
 {
-  return json_numbert(i2string(id));
+  return json_numbert(std::to_string(id));
 }