Use `Cilint` in `CInt64`

[michael-schwarz]

Since we have given up compatibility with original CIL a while ago, this simplifies the handling of constants by not relying on their textual representation.

[sim642]

Or maybe use Zarith directly? As an intermediate step maybe reimplement the Cilint module using Zarith inside (if it's used abstractly enough in the rest of CIL)?