AAD struct

Use a struct instead of the `sender_id`, `recipient_id` and `info` arguments needed for the *before* functions.

Provide a simple default, perhaps recipient_id being the public key and info simply being "rust-signcryption" for interop debugging. Also provide a Aad::empty() function to ignore the default fields.

Start by locating the before functions that currently accept sender_id, recipient_id, and info, then trace how those arguments are used for authenticated associated data. Done means those functions use an AAD struct with the requested default values and an Aad::empty() option; the payload names no specific files or tests to run.

Written by the indexing model from the issue text.