Replacing direct references to the arguments object of functions with rest
parametres prevents the function from being deoptimized while still allowing
use of arbitrary arguments. This may also fix some minor memory leaks related
to mishandling the arguments object.
We had a lot of discussion in Dev and a somewhat-close poll, but in
the end "Chat" was a better name than "Messages", and also has the
advantage of being shorter (which is nice for Chat.html and
Chat.plural which should be short).