Task executor preference (#2187)

* Initial revision of task executors proposal

* Cleanup and remove unclear pieces of design

* add missing withTaskExecutor in snippet

* Update NNNN-task-executor-preference.md

* remove ordering promises; they are not strictly guaranteed

* preference also affects default actors

* introduce the notion of TaskExecutor, in order to handle default actors

* change title

* add some warnings about when to use this proposal

* arrow typo in diagram

* Fix typo (#11)

Fixed a little typo

* Fixed typo. (#12)

* Update missed section; SerialExecutors are NOT TaskExecutors by default

* Explain Task(on actor) a bit more -- in future directions

* fix typo in async let example

* add unownedTaskExecutor API

* minor rewording in sentence

* document that executor can be both serial and task executor

* add minor notes to serial+task executor section

* Apply suggestions from code review

Co-authored-by: Doug Gregor <[email protected]>

* Update NNNN-task-executor-preference.md

* Cleanup a comment

* Update proposals/NNNN-task-executor-preference.md

Co-authored-by: John McCall <[email protected]>

---------

Co-authored-by: Genaro-Chris <[email protected]>
Co-authored-by: Wade Tregaskis <[email protected]>
Co-authored-by: Doug Gregor <[email protected]>
Co-authored-by: John McCall <[email protected]>

Author: 5 people