SI-1448 Deprecate conversion of numeric values types by casts

**Attention**: This is an alternative PR to #3294. Do **NOT** merge both.

Casts of AnyVals had inconsitent behavior depending on whether they were boxed:

    1L.asInstanceOf[Int]        // -> 1
    (1L: Any).asInstanceOf[Int] // -> ClassCastException

This PR fixes this by deprecating casts for conversion. I.e. in future scala versions, both calls will fail (although the first at compile time).

PR #3294 contains some discussion why this version might be desirable above the other.

Still partesting.