Rewriting MessageString/MessageBytes

[co...@costin.dnt.ro]

Hi,

I'm sorting MimeHeaders and friends, it seems MessageString/MessageBytes
are in a very bad shape - a lot of dark char<->byte conversion we can't
control.

Same for MimeHeaderFiled - it's not only complex but many operations are
in fact very expensive ( for exampe - date conversion creates Strings,
etc). 

I think evolution will not work for those - I will remove everything and
implement them again with only the required code. If someone understand
or use the current code - please speak now. 

( probably the change will have no other effect except improving the
performance a bit and cleaning up - but of course, it may add bugs )

Costin