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.

After checkfile update to accomodate deprecation warnings, all partests succeed. However, run/t7868 does now emit a deprecation warning due to the code generated by the pattern matcher (no difference in matching behaviour is observed). If we proceed with this option, this warning will have to be fixed in the pattern matcher.

**UPDATE** This does not deprecate casts from `null` to a value type.