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/GPBProtocolBuffers_RuntimeSupport.h

diff --git a/objectivec/GPBProtocolBuffers_RuntimeSupport.h b/objectivec/GPBProtocolBuffers_RuntimeSupport.h
index 7fd7b4c..fea75b9 100644
--- a/objectivec/GPBProtocolBuffers_RuntimeSupport.h
+++ b/objectivec/GPBProtocolBuffers_RuntimeSupport.h
@@ -34,8 +34,7 @@
 #import "GPBProtocolBuffers.h"
 
 #import "GPBDescriptor_PackagePrivate.h"
-#import "GPBExtensionField_PackagePrivate.h"
-#import "GPBExtensionRegistry.h"
+#import "GPBExtensionInternals.h"
 #import "GPBMessage_PackagePrivate.h"
 #import "GPBRootObject_PackagePrivate.h"
 #import "GPBUtilities_PackagePrivate.h"