Closes https://github.com/inducer/arraycontext/issues/70.

* add a leaf_class parameter to flatten

* add some docs to flatten for leaf_class

* allow specifying leaf class in recursive map and map-reduce

* revert broken changes to decorators

* add leaf_class to decorators

* add tests for [multi]mapped_over_array_containers

Co-authored-by: Andreas Klöckner <inform@tiker.net>

Kaushik Kulkarni authored Dec 23, 2021 and Andreas Klöckner committed Dec 24, 2021

`compile` only supported compiling callables that returned array
containers. Extends the logic to support compiling callables that simply
return thawed arrays.

Refactor PytatoActx.compile for usability by distributed