Do not box Ints.

Contrarily to the previous commit, where non-boxed values could
still always be identified in an accurate way, in this commit
we cannot do that anymore.

Since both Ints and Doubles are non-boxed as numbers, and their
values overlap, we cannot always tell whether a value was
initially an Int or a Double. Specifically, all integer numbers
will answer true to isInstanceOf[Int] *and* isInstanceOf[Double].
Hence, all Ints are also Doubles, and some Doubles are also Ints.