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/GPBUnknownField.m

diff --git a/objectivec/GPBUnknownField.m b/objectivec/GPBUnknownField.m
index 22ed66a..0e29bde 100644
--- a/objectivec/GPBUnknownField.m
+++ b/objectivec/GPBUnknownField.m
@@ -31,7 +31,7 @@
 #import "GPBUnknownField_PackagePrivate.h"
 
 #import "GPBArray.h"
-#import "GPBCodedOutputStream.h"
+#import "GPBCodedOutputStream_PackagePrivate.h"
 
 @implementation GPBUnknownField {
  @protected