Use an AEAD cipher instead of AES-CBC + HMAC-SHA1

[DemiMarie]

cookie-rs currently uses AES-256-CBC and then HMAC-SHA1 in encrypt-then-MAC. This is secure, since AES-256-CBC and HMAC-SHA1 are both individually secure and because they are used in encrypt-then-MAC.

However, neither AES-256-CBC nor HMAC-SHA1 is fast, even when AES is hardware accelerated (due to the serial nature of CBC mode). ChaCha20-Poly1305 is much faster.

Again, cookie-rs's current crypto is not insecure, just slow.

[alexcrichton]

Ideally this'd actually be configurable rather than only hardcoding one and calling it a day, but I'm somewhat hesitant to deviate away from the source of this, Rails, which is probably much more experienced than I :)

[frewsxcv]

Opened a PR: #68.

[briansmith]

There's another PR for this: #76.