Remove ChaChaXCore types

[dhardy]

In discussion of API bloat with @newpavlov, it became clear that we don't need these types.

The only known use is rand::rngs::ThreadRng, yet there's no reason that can't use ChaChaCore (already a public item).

[newpavlov]

If the field will be private this impl can be removed.

[dhardy]

The point being that we don't need trait CryptoGenerator at all. The trait is (I believe) only used by ReseedingRng, so lets start with that. (Another PR will be needed to update to the new rand_core soon anyway.)

[dhardy]

I think it better to merge this then make a new PR for rust-random/rand_core#68.

[dhardy]

Aside: it would be nice to expose the StreamCipherSeekCore methods (or at least get_block_pos for variant Legacy) without the cipher feature enabled.

What is the preferred way to do this — a simple unparametrised method? Or provide a copy of StreamCipherSeekCore (minus the trait bound) without feature "cipher"?

[newpavlov]

I think we can add public inherent methods and then use them in implementation of StreamCipherSeekCore.

[tarcieri]

Yeah, just add an inherent method with the same name/type signature as the trait. You can always disambiguate via UFCS

[dhardy]

Done. UFCS shouldn't be needed since inherent methods always have priority and the in/out types are the same...

... expect for $ChaChaXRng::set_block_pos which uses type BlockPos = U32x2. Shall I make this u64 instead to match the others?

[dhardy]

I believe that makes the most sense, so done (last two commits).

[newpavlov]

Note the failing CI.

[dhardy]

Why does ChaChaCore cfg-gate its state?

    #[cfg(any(feature = "cipher", feature = "rng"))]
    state: [u32; STATE_WORDS],

Is it supposed to compile to an empty struct if neither of these features are used?