Conversion to perform addition on two integer literals of type :int.
The call INT_ADD_CONV `c1 + c2` where c1 and c2 are integer literals
of type :int, returns |- c1 + c2 = d where d is the canonical integer
literal that is equal to c1 + c2. The literals c1 and c2 may be of the
form &n or -- &n (with nonzero n in the latter case) and the result will
be of the same form.
Fails if applied to a term that is not the sum of two permitted integer
literals of type :int.
# INT_ADD_CONV `-- &17 + &25`;;
val it : thm = |- -- &17 + &25 = &8