This is a rather large change because it touches public APIs, so I want to include all possible changes to make it easier for our current users to migrate in one go.

Notable changes:
* Add a mechanism that allows to restrict the visibility of functions that accept cleartext keys.
* s/Aead/AEAD/, s/Mac/MAC/, s/ComputeMac/ComputeMAC/, s/VerifyMac/VerifyMAC/, s/PublicKeySign/Signer/, s/PublicKeyVerify/Verifier/
* s/aead.GetPrimitive/aead.New/, etc.
* s/aead.RegisterStandardKeyTypes/aead.Register/, etc.

PiperOrigin-RevId: 215424138
GitOrigin-RevId: 3e9452ec61d04294ab0afdc9c395fe320cefa6a7