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.

objectivec/GPBMessage.m

diff --git a/objectivec/GPBMessage.m b/objectivec/GPBMessage.m
index fdb695e..d5e8d37 100644
--- a/objectivec/GPBMessage.m
+++ b/objectivec/GPBMessage.m
@@ -35,7 +35,7 @@
 
 #import "GPBArray_PackagePrivate.h"
 #import "GPBCodedInputStream_PackagePrivate.h"
-#import "GPBCodedOutputStream.h"
+#import "GPBCodedOutputStream_PackagePrivate.h"
 #import "GPBDescriptor_PackagePrivate.h"
 #import "GPBDictionary_PackagePrivate.h"
 #import "GPBExtensionInternals.h"