Cleanup: IO, inline namespace declarations

Replace nested `namespace blender { namespace io {`
with `namespace blender::io {`.

No functional changes.

source/blender/io/common/IO_abstract_hierarchy_iterator.h

@@ -51,8 +51,7 @@ struct Object;
 struct ParticleSystem;
 struct ViewLayer;
 
-namespace blender {
-namespace io {
+namespace blender::io {
 
 class AbstractHierarchyWriter;
 class DupliParentFinder;
@@ -356,5 +355,4 @@ class AbstractHierarchyIterator {
   ExportChildren &graph_children(const HierarchyContext *parent_context);
 };
 
-}  // namespace io
-}  // namespace blender
+}  // namespace blender::io

source/blender/io/common/intern/abstract_hierarchy_iterator.cc

@@ -44,8 +44,7 @@
 
 #include "DEG_depsgraph_query.h"
 
-namespace blender {
-namespace io {
+namespace blender::io {
 
 const HierarchyContext *HierarchyContext::root()
 {
@@ -755,5 +754,4 @@ bool AbstractHierarchyIterator::should_visit_dupli_object(const DupliObject *dup
   return !dupli_object->no_draw;
 }
 
-}  // namespace io
-}  // namespace blender
+}  // namespace blender::io

source/blender/io/common/intern/object_identifier.cc

@@ -26,8 +26,7 @@ extern "C" {
 #include <cstring>
 #include <sstream>
 
-namespace blender {
-namespace io {
+namespace blender::io {
 
 ObjectIdentifier::ObjectIdentifier(Object *object,
                                    Object *duplicated_by,
@@ -112,5 +111,4 @@ bool operator==(const ObjectIdentifier &obj_ident_a, const ObjectIdentifier &obj
   return obj_ident_a.persistent_id == obj_ident_b.persistent_id;
 }
 
-}  // namespace io
-}  // namespace blender
+}  // namespace blender::io