* improved nested insert execution logic
* integrate error path, improve executing 'withExp' and improve tests
* add more readable types in '/Resolve/Insert.hs'
* set conflict context just before executing WITH expression
Better SQL generation for select queries (the query plans will be the same but much more readable). This closes some long standing issues (#6, #121, #278).
Insert trigger function: If query affects no rows then return `null`
Insert trigger function is modified to have
`IF r IS NULL THEN RETURN null; ELSE RETURN r; END IF;` in return statement.
* fix primary key changing on upsert, fix#342
* add 'update_columns' in 'on_conflict' object, consider 'allowUpsert'
* 'ConflictCtx' type should respect upsert cases
* validation for not null fields in an object
* add 'on_conflict' condition to allow upsert mutation, closes#105
* check for empty unique or primary key constraints
* add 'on_conflict' condition test cases and introspection test case
* update 'conflict_action' enum values' description
