Can't have `NSString` and `NSNumber` as values in an `NSDictionary`

[mwcampbell]

I don't see a way to create an NSDictionary with an NSString and an NSNumber as values, because the public Rust API for NSDictionary requires values to be Owned references, and this isn't possible for NSString and NSNumber. This blocks me from posting announcement notifications in the AccessKit macOS adapter, unless I use an unsafe workaround.

[madsmtm]

Indeed, this is an issue.

In #264 I'm experimenting with adding ownership parameters to NSDictionary as well (so having NSDictionary<Key, Value, KeyOwnership = Shared, ValueOwnership = Shared>, which should solve this.

[madsmtm]

This is partially fixed now, since NSMutableDictionary::setObject_forKey is now available regardless of the dictionary's key and value ownership.

Though I haven't updated the helper implementations, so e.g. NSMutableDictionary::insert is currently unsound.