HeaderDoc support in the library and generated sources

- Convert most of the core library headers over to HeaderDoc format.
- Switch the generated comments over to HeaderDoc.
- Create GPBCodedOutputStream_PackagePrivate and move some things into there
  that should be more internal.
diff --git a/objectivec/GPBExtensionInternals.m b/objectivec/GPBExtensionInternals.m
index 634c336..7d0dcb2 100644
--- a/objectivec/GPBExtensionInternals.m
+++ b/objectivec/GPBExtensionInternals.m
@@ -33,7 +33,7 @@
 #import <objc/runtime.h>
 
 #import "GPBCodedInputStream_PackagePrivate.h"
-#import "GPBCodedOutputStream.h"
+#import "GPBCodedOutputStream_PackagePrivate.h"
 #import "GPBDescriptor_PackagePrivate.h"
 #import "GPBMessage_PackagePrivate.h"
 #import "GPBUtilities_PackagePrivate.h"