SI-1448 Deprecate conversion of numeric values types by casts

Casts of AnyVals had inconsitent behavior depending on whether they
were boxed:

    1L.asInstanceOf[Int]        // -> 1
    (1L: Any).asInstanceOf[Int] // -> ClassCastException

This commit fixes this by deprecating casts for conversion. I.e. in
future scala versions, both calls will fail (although the first at
compile time).