Check for overflows in these functions:
- `operator+`
- `operator-`
- `operator+=`
- `operator-=`
Note: `operator-`, `operator/` and `operator%` don't need overflow checks, because by design these can never overflow, as we exclude `std::numeric_limits<int64_t>::min()` from the valid script integer range.
Depends on D17000 and D16997.