Integration of bitShift: speed up

 
There is another change I'd like to see integrated.
That is http://bugs.squeak.org/view.php?id=7109.

If you want to compare to existing code, be aware I renamed
#cCoreBytesRshiftCount:n:m:f:bytes:from:len:to:len:
to:
#cCoreBytesRshiftCount:n:f:bytes:from:len:to:len:
the m: was useless, at least in new implementation...

cheers

Nicolas