Add `CFRetained::into_super` or similar

[madsmtm]

It would be nice to have a way to "upcast" things like CFMutableDictionary to CFDictionary, without having to go do (&dict).into() (which requires an extra retain/release).

Motivating use-case: things like IOServiceMatching returns a mutable dict which is consumed as an immutable dict by things like IOServiceGetMatchingService, see #744.