by default; a new optional argument restores the old behavior
(but the default behavior is consistent with the old docs and with
the vast majority of existing uses)
The implementation is ugly for performance reasons. A new primitive
`prop:arity-incomplete' property determines when to return #f for
`procedure-arity-includes?' in default mode. A nicer implementation
would be to redefine `procedure-arity-includes?' at the kw-proc level,
but the bytecode optimizer's and JIT's treatment of the built-in
`procedure-arity-includes?' is important. The implementation choice
could be revisited after cross-module inlining is implemented.
Closes PR 11978
