covariant returns for CharBuffer.subSequence

[Alan Bateman]

Martin Buchholz wrote:
> :
> As often happens with me, this change is ever-expanding out of scope.
> If we have consensus on the covariant returns for other Buffers,
> I am willing to add these to the change.
>   
I don't mean to twist your arm :-)   As this is API change there is a 
bit of process work which is why I suggested we do it as one bugID 
rather than several. If you'd prefer I can do the updates to the buffer 
classes, get the process work started next week, and let you focus on 
the String updates.

-Alan.