Beta quality drop of Objective C Support.

- 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.

objectivec/Tests/GPBConcurrencyTests.m

diff --git a/objectivec/Tests/GPBConcurrencyTests.m b/objectivec/Tests/GPBConcurrencyTests.m
index e500ad7..daf75e7 100644
--- a/objectivec/Tests/GPBConcurrencyTests.m
+++ b/objectivec/Tests/GPBConcurrencyTests.m
@@ -196,7 +196,8 @@
   NSArray *threads = [self createThreadsWithSelector:sel object:messages];
   [self startThreads:threads];
   [self joinThreads:threads];
-  GPBExtensionField *extension = [UnittestRoot optionalForeignMessageExtension];
+  GPBExtensionDescriptor *extension =
+      [UnittestRoot optionalForeignMessageExtension];
   for (TestAllExtensions *message in messages) {
     XCTAssertFalse([message hasExtension:extension]);
   }