Avoid N+1 in postgresql migrations (#1612)

* wip

* Handle existence + test

* Move diffing logic over

* Replace the impl

* expand test

* what da hell

* tracing

* hell yeah

* remove tracing

* format

* Move last two migration fns into Internal.Migration

* docs + re-export new migrateEntitiesStructured

* bump version + add changelog

* format

* Handle duplicate FK constraints

* Add test for duplicate FK constraints

* Switch to pg_catalog for FK constraints

* format

* Restyled by cabal-fmt

---------

Co-authored-by: Restyled.io <[email protected]>

Author: hdgarrood

persistent-postgresql/ChangeLog.md

@@ -1,5 +1,11 @@
 # Changelog for persistent-postgresql
 
+# 2.14.1.0
+
+* [#1612](https://github.com/yesodweb/persistent/pull/1612)
+  * Speed up migrations by avoiding N+1 queries.
+    You can now migrate a large set of entities much faster, by using the new `migrateEntitiesStructured` function.
+
 # 2.14.0.1
 
 * [#1610](https://github.com/yesodweb/persistent/pull/1610)