DynamoDB: DeleteItem command and a number of smaller enhancements

- Operational DeleteItem command (needs testsuite)
- An experimental FromDynItem and ToDynItem interface, a-la aeson
- A few missing exports and some performance fixes
- More general awsIteratedSource/List implementation

One TODO item left is to rewrite the existing awsIteratedSource/List functions in terms of the more general versions created here. 
Please avoid unnecessary code line changes.