Add cond and unquote splicing.

This needed some ugly meatball surgery, removing a lot of AtomishForm wrapping and then
fixing other code to deal with accepting unwrapped forms. Some kinds of AtomishCommated's
will probably explode, though I have not managed to exercise any such explosions - they
would need to be artificially provoked.

Unquote splicing is still a bit raw, and it has disturbed normal unquotation a little as
well.