[Deferred] SE-0058 Allow Swift types to provide custom Objective-C representations

The review of SE-0058 "Allow Swift types to provide custom Objective-C representations" ran from April 4...11, 2016. The proposal has been deferred from Swift 3. We agree that it would be valuable to give library authors the ability to bridge their own types from Objective-C into Swift using the same mechanisms as Foundation. However, we lack the confidence and implementation experience to commit to `_ObjectiveCBridgeable` in its current form as public API. In its current form, as its name suggests, the protocol was designed to accommodate the specific needs of bridging Objective-C object types to Swift value types. In the future, we may want to bridge with other platforms, including C++ value types or other object systems such as COM, GObject, JVM, or CLR. It isn't clear at this point whether these would be served by a generalization of the existing mechanism, or by bespoke bridging protocols tailored to each case. This is a valuable area to explore, but we feel that it is too early at this point to accept our current design as public API.