Canonicalize system names to strings in asdf.asd
ASDF has been canonicalizing system names to string with COERCE-NAME since its very beginning. Working at Google, which uses automatic formatters to keep all its source code as canonical as possible, convinced me that keeping things canonical is better. I've been applying this style to my own systems and recommending it to others. stassats noticed that asdf.asd was failing to lead by example -- so I'm fixing it accordingly.