Figure out whether CF collections should be generic

[madsmtm]

Relevant for the types CFArray, CFBag, CFBinaryHeap, CFDictionary, CFSet and maybe CFTree.

For a bit of prior art, core-foundation, the predecessor to objc2-core-foundation, declares these as generic, but the headers themselves do not.

This is especially problematic because it prevents any function that takes e.g. &CFArray or &CFDictionary from being marked as safe, because those arrays/dictionaries can contain anything (while the function expects of a certain type, or at least just that the inner type is a CFType). Returned arrays/dictionaries are also harder to use, because you have to manually ensure that they contain the right value.

As said, the headers do not provide this type information directly, but perhaps we can enrich it ourselves?

Basically, it comes down to a choice between:

• Making every element accessor unsafe, and forcing users to consider the type there.
• Making element accessors safe (but dependent on the element type), and making conversion/construction of a typed collection from an untyped collection unsafe.

This probably blocks #692 and #693.