walk_path: Return the name of the innermost accessed table as well
Add a class to generate triggers and trigger functions

This is untested at the moment, because I'm not around a PG DB.
Perform DELETE triggers before the DELETE
Include trigger generation in the API docs
Add TODOs
Always compareto primary key values
If a path end in a one-to-many relationship, use the remote column name
Add triggers to the correct table for one-to-many relationships
Shorten the trigger generation a bit by ignoring paths ending in columns

Relationships already contain selections on the various primary keys. If
a column value itself changes, it will already be caught by that.
Rename new_or_old where it's replaced
Generate triggers for all entities