Make body encoding return bytes instead of a buffer
Do body and header encryption from within the inner header functions.
At this point, the system should handle extended Mixmaster messages but
sadly, it does not.  There is a bug in the reference 3.0.2e
implementation that means IVs used to encrypt body, headers and inner
are unintentionally different.  This behaviour is difficult to reproduce
using Go and to do so would be to propagate a bug that should be
corrected.  Further development is unlikely unless this is resolved.