Pointer cleanup (#242)

- Made greater use of smart pointers, eliminating many instances of manual memory management
- Do not use pointers at all for many non-polymorphic classes
- Assorted other code cleanup

backends/merge/None.cpp

@@ -108,9 +108,9 @@ namespace glabels
 		///
 		/// Read next record
 		///
-		Record* None::readNextRecord()
+		Record None::readNextRecord()
 		{
-			return nullptr;
+			return NullRecord();
 		}
 
 	} // namespace merge