Publish `chacha20` crate which is available `neon` feature

[sorairolake]

They want to use this feature in shadowsocks-crypto crate, but this is not yet available. When will this feature available version be published?

see also: shadowsocks/shadowsocks-crypto#19

[tarcieri]

The unaddressed (potential) work item is gating the feature on a cfg! attribute instead of a crate feature, like we have moved to in most other crates, as was mentioned here.

This prevents transitive dependencies from activating the feature and ensures it's controlled completely by the toplevel binary.