Header change

A five-bit field is unusual; fields are usually 1 4 or 8.  It seems to
me that it's only five because MB ME CF take up the other three. :)

I'd like to suggest the following layout change
       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |  M M C        |
      |V V V V x x x x|x B E F T T T T|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MB ME and CF are are in the current spec.  The four-bit TTTT is
the TYPE_T field as in the current spec.  The four-bit VVVV field
is the version number, but not as defined in the current spec.
Use b'1000' as the first version; b'1100' as the second version,
and so on.  Actually, don't define the other versions yet; this
allows maximum "games" to be played with the reserved "x" bits.
In version 1, the "x" bits are reserved and must be zero.

This avoids shifting the version, putting all version-specific semantics
(at least those known so far) in a single byte, coalsces the expansion
fields, and so on.

Comment?

On padding:  I think it would be more clear if a single section
described the "padding" algorithm, rather than cut-and-paste wording
in each of 3.2.13 etc.  I like "padded to nearest multiple of four
not less than the message size," but that's wordsmithing.

        /r$