Issue Description:

Currently Swift's Metal overlay for MTLCopyAllDevicesWithObserver has the following signature:

public func MTLCopyAllDevicesWithObserver(handler: @escaping MTLDeviceNotificationHandler) -> (devices:[MTLDevice], observer:NSObject)

The observer is returned as an NSObject. However, the underlying Obj-C API returns it as an id<NSObject>, which is imported into Swift as NSObjectProtocol. This mismatch means the overlay currently needs to force cast to NSObject.

Ideally we should change the Swift overlay such that it's returned as an NSObjectProtocol, or possibly even Any.