Using ASCII values for keys is a bad idea since it makes them vastly
easier to guess. Instead, use the same method as the examples in the
golang.org/x/crypto/nacl package to load keys from a hex value.

Changing the key required updating the ciphertext in many of the
examples.

I am still worried about the fact the examples ask the user to
authenticate messages; authentication isn't trivial, and to be honest
it may be better to steer people to a higher level primitive like
secretbox, unless people really need AES.

Fixes #21012.

Change-Id: I8d918cf194694cd380b06c2d561178167ca61adb
Reviewed-on: https://go-review.googlesource.com/48596Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>