Two minor incompatible changes as a result of two-tier compilation

Surpringly such a large overhaul of the VM has not resulted in many incompatible changes. There are two I'm aware of:

• Quotations are immutable now -- I've mentioned already.
• Passing f to call or if is an error now. Being able to call f was a hold-over from the cons cell days when f was the empty quotation. These days this has required an unnecessary type dispatch, and it is what I consider a rather awkward pun. So this is a type error now.