Cmm Commentary Update

I updated the Cmm Commentary page, at last.  If you have time, please  
take a look and let me know whether you would like me to fill out any  
data.  Although the page is very large for a wiki, there was too much  
ground to cover for me to get all the details: I wanted to  
concentrate on differences between the Cmm implementation and the C--  
specification, the implementation itself in GHC and how that works  
with the CodeGen and NCG, and generally tie things together  
properly.  The page is weak on Cmm Calls and the real information on  
Primitive Operations is spread out a little.  I wanted to put in a  
section on implementing new primitive operators but I have run out of  
time for the moment--the notes are waiting and I am testing a few  
things, particularly oddities of exceptions which seem to need  
individual coding since Cmm lacks flow controls like `also` and `cut  
to`.  Anyway, I hope it helps.

Cheers,
Pete