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).

Update checkfiles to accomodate deprecation warnings. (Note that
run/t7868 does not observe any difference in matching behavior)