Normalise AEAD direct interface

Normalise AEAD direct interface

The AEAD interface had three problems:

- Inconsistent prefix (now fixed, to "crypto_aead").
- Inconsistent ordering of arguments (now the mac is after the output
  text).
- Redundant API without the additional data (now removed).

The result is less than satisfactory to be honest.  If it were just me I
would delete the direct interface entirely, because the streaming one is
almost easier to use...

...save one crucial detail: the choice of the exact algorithm.  The
streaming interfaces offers three init options, each with its pros and
cons.  Users need a default, and it shall be XChacha20.  Those who know
what they are doing can easily use the streaming API anyway.