* fix nested insert with returning computed fields gives error, fix#3609
* revert using ordered hashmaps, sort columns based on ordinal postion
* fix 1. keys order 2. json/jsonb column value in nested insert returning
* add a note for sorted columns
* cast 'VALUES' expression as table row type
* use single CTE expression for generating returning for nested inserts
This fixes an issue where queries could incorrectly be considered
reusable if a variable was used in two positions: one where it affected
SQL generation and one where it did not.