Experimenting with different adder implementations

The experiments were performed using the following example:
```
 #ifndef SIZE
 #define SIZE 100
 #endif

 #ifndef UNSAT
 int main()
 {
   int a[SIZE], b[SIZE];
   for(int i=0; i < SIZE; ++i)
     __CPROVER_assert(a[i] + b[i] > a[i], "");
 }
 #else
 #include <limits.h>

 int main()
 {
   int a[SIZE], b[SIZE];
   for(int i=0; i < SIZE; ++i)
   {
     __CPROVER_assert(
       b[i] <= 0 ||
       (a[i] >= (INT_MAX >> 28) || b[i] >= (INT_MAX >> 28)) ||
       (a[i] <= (INT_MIN >> 1) || b[i] <= (INT_MIN >> 1)) ||
       a[i] + b[i] > a[i], "");
   }
 }
 #endif
```
See https://tinyurl.com/adder-comparison
(https://docs.google.com/spreadsheets/d/1ckrIwxe5QVUu12rs3OaY3o7CQ-UqFnEWvcEOf8cw580/)
for results.

Author: tautschnig