Removed unnecessary overhead from Object::copy().

The shared_ptr machinery has a pretty significant overhead when copying small objects - this change gives almost a 50% reduction in runtime in a benchmark which copies many small data objects (where lazy-copy-on-write defers most of the real work anyway).

There were a few unecessary overheads :

- Allocating the CopyContext on the heap.
- Using a shared_ptr
	- Allocates another heap object for use as a reference count.
	- Uses atomic operations to increment and decrement the reference count.

Author: johnhaddon

src/IECore/Object.cpp

@@ -312,8 +312,8 @@ size_t Object::MemoryAccumulator::total() const
 
 ObjectPtr Object::copy() const
 {
-	boost::shared_ptr<CopyContext> c( new CopyContext );
-	ObjectPtr result = c->copy( this );
+	CopyContext c;
+	ObjectPtr result = c.copy( this );
 	return result;
 }