crypto, string_bytes: treat `buffer` str as `utf8`

Do not treat crypto inputs as `binary` strings, convert them to Buffers
using `new Buffer(..., 'utf8')`, or using newly updated StringBytes
APIs.

PR-URL: https://github.com/nodejs/node/pull/5522


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>