Implement https://tools.ietf.org/html/rfc7366 in response to https://tools.ietf.org/html/rfc7457#section-2.4

This requires creating modified versions of the functions #'encrypt-and-send and #'ciphertext-to-compressed that handle mac-then-encrypt rather than encrypt-then mac.
In addition, add mechanisms to the hello messages to facilitate negotiating this extension.