src/c-types-parse.lisp (parse-declarator): Refactor argument list parsing.

src/c-types-parse.lisp (parse-declarator): Refactor argument list parsing.

The `argument-list' parser is now less monolithic: the work of parsing
individual arguments is given to new functions `arg-decl' and
`argument'.

Also, the interface between `argument-list' and its parent
`postfix-lparen' is changed.  Instead of simply returning its argument
list, `argument-list' now returns a function which can be applied to a
base type to produce the appropriate function type.

None of this changes any externally observable behaviour.