src/c-types-proto.lisp, src/c-types-impl.lisp: Qualifier name protocol.

src/c-types-proto.lisp, src/c-types-impl.lisp: Qualifier name protocol.

Add a new generic function `c-qualifier-keyword' to convert a Lisp
qualifier (confusingly, a Lisp keyword) into its C equivalent.
Currently, this just lowercases the name, but future keywords won't
necessarily have such simple mappings.

Also add `c-type-qualifier-keywords' to collect and convert the list of
keywords attached to a type, and use it in the type printing functions.
Take the opportunity to add more discretionary newlines in the pretty-
printing.