- move deprecated interfaces down to bottom of TOC, update verbiage

- move deprecated interfaces down to bottom of TOC, update verbiage
- more docs for engine, pool, DDL events
- update DDL sequences documentation to use events
- update DDL() docstring to refer to execute_if()
- document parameters for DDLElement.execute_if()
- add retval=True flag to Engine.on_before_execute(), on_before_cursor_execute().
wrap the function if retval=False, check for appropriate usage of the flag, add
tests.
- remove ScopedSession.mapper and tests entirely
- remove ExtensionCarrier and tests
- change remaining tests that use MapperExtension to use MapperEvents