The CocoaPod (and source) have always support tvOS, but adding the project
makes it easier to run the tests and debug if there ever were an issue.

(not adding a watchOS project at the moment because Apple doesn't provide
 XCTest, so all it could do is build the library. We still support that,
 just can't "test" it.)

- Fix up -copyWithZone: to not leave the two registries sharing
  some of the storage by using -addExtensions:.
- Improve -addExtensions: to clone the sub dict when there is
  nothing to merge into.
- A ExtensionRegistry unittests.
- Update project schemes to not have extra things in perf scheme.

- Let Xcode 8 update settings on the projects/schemes.
- Migrate Swift tests to Swift 3 syntax.
- Update the build/test script:
  - Require Xcode 8 (because of the Swift 3 requirement for tests)
  - Update the devices to what Xcode 8 has (8.x simulator seem to fail even
    though they can be downloaded in Xcode 8)
- Update the travis images to ones with Xcode 8.

Remove the stale reference to test no longer around, and keep the performance test limited to just performance tests.

- Let Xcode update the projects, schemes, and info.plists.
- Add workaround for shallow analyzer issues in current Xcode versions (deep analyze gets things correct).
- Tweak the Swift based tests to avoid warnings from Xcode 7's XCTest using optionals for autoenclosure results.
- No longer tag the ObjC iOS travis test as flaky, xctool seems to manage the simulator pretty well.

- Add more to the ObjC dir readme.
- Merge the ExtensionField and ExtensionDescriptor to reduce overhead.
- Fix an initialization race.
- Clean up the Xcode schemes.
- Remove the class/enum filter.
- Remove some forced inline that were bloating things without proof of performance wins.
- Rename some internal types to avoid conflicts with the well know types protos.
- Drop the use of ApplyFunctions to the compiler/optimizer can do what it wants.
- Better document some possible future improvements.
- Add missing support for parsing repeated primitive fields in packed or unpacked forms.
- Improve -hash.
- Add *Count for repeated and map<> fields to avoid auto create when checking for them being set.