chacha20: apply unreachable_pub lint

[newpavlov]

Re-exports Nonce and ChaCha20LegacyCore.

In a future breaking release we probable should just make legacy, rng, and xchacha modules public instead of re-exporting them.

Closes #569

[newpavlov]

Huh, it looks like Intel has started to block curl downloads. The archive is downloaded fine using browser, but I get an empty file using curl and wget. Simply adding a user agent did not help.

UPD: According to this issue, it looks like a JS-based challenge was added for the download. I guess we should just upload a copy of the tarball to RustCrypto/actions.

[tarcieri]

I can look into fixing the avx512 job and getting everything migrated to the shared action to install the SDE in https://github.com/RustCrypto/actions

[newpavlov]

Note that we do not use the shared job here, so the chacha20 config would have to be updated as well.