This change adds support for the ChaCha20-Poly1305 AEAD to crypto/tls,
as specified in https://tools.ietf.org/html/rfc7905.

Fixes #15499.

Change-Id: Iaa689be90e03f208c40b574eca399e56f3c7ecf1
Reviewed-on: https://go-review.googlesource.com/30957
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>