Implement From<_> for CFNumber
All the `from_` methods on `CFNumber` can be expressed with implementing the `From` trait and thus enable it to be used a bit more ergonomic in some cases. And feels more idiomatic IMO.
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Add CFPropertyList and methods for casting to subclasses
https://developer.apple.com/documentation/corefoundation/cfpropertylist-rif specifies the following types as valid subclasess of a property list:
CFData, CFString, CFArray, CFDictionary, CFDate, CFBoolean, and CFNumber.
So I add a trait, `CFPropertyListSubClass`, and implement it for the valid types. Then add the `CFPropertyList` type and make it work as much as possible as the other `TCFType`s. However, it can't implement that trait, since it can't implement the static method `type_id`.
The main point of adding this was to then get to the convenience methods `to_CFPropertyList` and `downcast` that allows safe casting between the subclasses and the superclass. This is handy in calls such as `SCDynamicStoreSetValue` and `SCDynamicStoreCopyValue` where the methods either accept or return a `CFPropertyListRef`. With this addition it becomes easy to write safe wrappers around those functions and make them accept proper, constrained types.
Disclaimer: I'm not yet super familiar with CF. So I can only hope this is safe and sound, but I think it is.
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Higher level dictionary find/contains_key
I'm not very familiar with Core Foundation yet, so tell me if this is not a very helpful addition.
Adding versions of find and contains_key that can take any `TCFType` and automatically converts it to the raw pointer of its corresponding ref type. It looks like dictionaries will mostly contain types implementing this trait, and thus these versions of the methods can make it a bit more ergonomic to use the dictionary. The best would of course be if we could return the correct type as well, but I guess that is not possible with the available information.
Better suggestions for the names are welcome :)
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Add runloop source and observer functionality
I had the need to add a source (from `SCDynamicStoreCreateRunLoopSource`). And since all the add/contains/remove operations were fairly trivial I did not see any reason to not complete them for timers, sources and observers at the same time.
Also adding the `CFRunLoopMode` type alias to make the signatures reflect the original API a bit more.
Lastly, removing underscore from `_info` since it's actually used.
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Implement From<&str> for CFString
I'm not sure if you have any strict policies on what types you usually accept as parameters to the safe wrappers. So I'm not sure if you want this or not. But my reasons are:
I create `system-configuration` and `system-configuration-sys` crates as bindings to that API. In the [`SCDynamicStore`] module a lot of the functions take a `CFStringRef` as argument. It would be more ergonomic if this safe wrapper type could deal with Rust strings directly, so I can do:
```rust
let store = SCDynamicStore::create("my-dynamic-store");
let dict = store.find("State:/Network/Global/IPv4");
```
But at the same time I don't want to make it harder to use for people who already sit on `CFString` objects. So with this new `From<&str>` impl my API can be used both with `&str` and `CFString` if I implement the methods as `<S: Into<CFString>>`.
[`SCDynamicStore`]: https://developer.apple.com/documentation/systemconfiguration/scdynamicstore?language=objc
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Linus Färnstrand
