Make the neon feature available for ChaCha20

Hello,

Today ChaCha20 (and by extension (X)ChaCha20-Poly1305) is painfully slow on arm64 devices.

NEON support was merged last year (#310) but is still not available for use.

Is it possible to publish a new version and update the docs so we can use the NEON version?

I can create a PR for the doc (adding a few explanation on how to enable NEON support) if necessary.

Many thanks!

[tarcieri]

As noted on #310, the reason it hasn't been released yet is that the PR gated it under a crate feature and we would like to move it to a cfg attribute for consistency.

See the chacha20_force_soft, chacha20_force_sse2, chacha20_force_avx2 cfg settings as examples.

[tarcieri]

I opened #317 to add the cfg attribute

Many thanks 🙏