Overflow

- warn when integers not fitting in 32bit are used into javascript ast
  (appends when compiling from 64bit host)
  - ints with the 32nd bit set (`0x8000_0000`) are detected as overflowing when on 64bit host (don't know what we should do here ?)
- `int_of_string` fixed : now accept `0x8000_0000` ...
- int32 arithmetic was not wrapped with `...|0`
```
Int32.add (Int32.add 0x7fff_ffffl 0x7fff_ffffl) 0x7fff_ffffl;;
- : int32 = 6442450941l
```