XS: Integer overflow

Numbers in javascript are floats, but this code produces what appears to be an integer overflow:

`./xst -e "let q = 2000000000; print(q); q -= (-2000000000); print(q);"`

Expected result:
```
2000000000
4000000000
```

Actual result (Windows build):
```
2000000000
-294967296
```

Actual result (Linux build):
```
2000000000
1705032704
```