Conversions from Int64 to Int don't check bounds correctly

[forell]

There are actually two problems. First, the bounds in Int64.toIntOpt are just plain wrong, and different from the other version of the method. Second, the Int type is implemented using OCaml's int in the interpreter, so its size is platform-dependent. It may be useful to expose the maximum and minimum integers in the standard library, and defining these values in one place should help implement integer conversions consistently.