Remove ReseedingRng

[dhardy]

• Added a CHANGELOG.md entry

Summary

Removes ReseedingRng, then reduces the remaining code. Closes #1721.

Motivation

This generic type appears to have little or no real use outside of ThreadRng. Removing it allows for several simplifications (also in rand_core).

Details

(This has a number of little commits so should ultimately be squashed.)

[newpavlov]

I think it's worth to also remove bytes_until_reseed and use block counter stored in the ChaCha core as part of this PR.

[newpavlov]

Keepchangelog recommends to use Added/Changed/Removed.

[dhardy]

None of the other recent titles match that... a job for another PR.

[dhardy]

Aha, #1686 uses rng_core.block_pos(). I don't see that method and get_block_pos() is only on the RNG, so I think we need to add a method (we don't use the cipher feature).

[newpavlov]

Yes. You can add it as part of RustCrypto/stream-ciphers#512.

[dhardy]

Ironically such a method would be one use for keeping the ChaCha*Core types.

[newpavlov]

The difference is that it's not a "core RNG" type. It's a chacha20-specific type which we expose either way for potential cryptographic applications with inherent methods useful for ThreadRng implementation.