added returning token for insert query

todo.md

@@ -1,13 +1,25 @@
 # Todo
 
 - move `prepare_*` methods from `dialect` to `query_compiler`
-- add `session_insert_builder` and `session_update_builder` (returning multiple statements)
+- add `insert_query_builder` and `update_update_builder` (returning multiple statements)
 - finish fetch eager many-to-many relations
 - implement lazy loading
 - implement polymorphic class hierarchies
-- finish `schema_repository` classes (move add/drop from `session` to `schema`)
+- finish `database` (schema_repository) class (move add/drop from `session` to `schema`)
 - implement a flag class for enumerations
 
+- PK generator
+1. Introduce execute_context
+2. Introduce execute_result
+3. Add `abstract_pk_generator` class
+4. Add `identity_pk_generator` class
+5. Add `sequence_pk_generator` class
+6. Add `table_pk_generator` class
+7. Add `manual_pk_generator` class 
+8. Extend `session::insert` logic to use pk generator
+9. Extend `postgres_connection::execute` to handle `returning` clause
+10. Add generator to `schema_node` or `table` class when attaching type
+11. Extend `query_builder` and `intermediates` with `returning` intermediate
 
 __Proposal for polymorphic classes:__