gcc will only inline functions if they are defined
before where they are used.  So this patch just moves
Encode and Decode up to the top of the file.

This can provide a couple percent improvement in speed.
(try
timeit.py -s 'import md5; m=md5.new(); d="a"*64'
'm.update(d);m.digest()'
and
timeit.py -s 'import md5; m=md5.new(); d="a"*0x100000'
'm.update(d)'
)

Decode is only used once, so allowing it to inline
actually reduces the code size, Encode is used twice
but its a pretty small function so it doesn't change
the size much either way..