This does not change the output yet, but it prepares the way for the
full implementation of the modern format, in which format and other
details are encrypted with pad and are part of the encoded output.

* onetime
  (Pad.OverPrepared): New exception class.
  (Pad._make_inner_headers): Renamed from Pad._make_fuzz.
  (Pad.__init__): Add new instance booleans _encoding and _decoding.
  (Pad.prepare_for_encoding, Pad.prepare_for_decoding): New methods.
  (Pad.convert): Revamp initialization logic as per above.
  (PadEncoder.__init__): Prepare pad for encoding.
  (PadDecoder.__init__): Prepare pad for decoding.