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.