Make legacy declarations private: macro guard, cipher

[gilles-peskine-arm]

Make legacy functions private by guarding their declaration with a macro.

As an example, do cipher.h (functions only, I didn't check if some macros and types should become private).

This is up for design review. There is no update to documentation yet. Some open questions:

• Is this approach ok? It's a quick win for not having legacy functions declared .
• Whether we want to update documentation at the same time we make things private, or later.
• Whether we want to add changelog entries, and whether to do it incrementally or all at once at the end.

PR checklist

• changelog provided | not required because:
• framework PR not required
• mbedtls PR Make legacy declarations private: macro guard, cipher mbedtls#9868 (breaks mbedtls)
• tests provided