Remove Clone for StdRng, new bound for ReseedingRng

[dhardy]

• Added a CHANGELOG.md entry

Since Clone support was removed for chacha20 (RustCrypto/stream-ciphers#462) we must remove Clone for StdRng.

For ReseedingRng, we could:

1. Make no changes. Effectively it would still lose Clone support with ChaCha*Core.
2. Add a restriction Rsdr: RngCore (i.e. that the reseeder is infallible) to allow seeding a new inner RNG on clone.
3. Try to reseed on clone, panic on failure. (I dislike this option.)
4. Remove Clone support entirely.

I opted for (4) since I suspect this feature has very little usage anyway. If users request that Clone support be restored we can re-evaluate based on the reported use-case and implement it in a patch release.