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

[gilles-peskine-arm]

It would be convenient to additionally annotate all function declarations between #if defined(MBEDTLS_DECLARE_PRIVATE_IDENTIFIERS) and the corresponding #endif with MBEDTLS_DECLARE_PRIVATE_FUNCTION which can be defined to expand to something like __attribute__((__deprecated__)). This would allow detecting the uses of private functions through compiler warnings, and gradually eliminating them by eliminating warnings.

We can add these annotations mechanically, once we've manually added the preprocessor guards.