Add executable models of CProverString functions

These models can be compiled and run and have the same semantics as implemented
by the string solver. They can thus be used to check that the string solver
gives the correct result by comparing the result of a concrete run with the
result obtained by cbmc.

Author: danpoe

jbmc/regression/jbmc-strings/cprover/CProverString.class

@@ -147,4 +147,12 @@ public static StringBuffer insert(
             StringBuffer sb, int offset, long l) {
         return sb.insert(offset, l);
     }
+
+    public static String toString(int i) { return String.valueOf(i); }
+
+    public static String toString(long l) { return String.valueOf(l); }
+
+    public static String toString(float f) { return String.valueOf(f); }
+
+    public static String toString(double d) { return String.valueOf((float)d); }
 }