P1897 r3 (#17)

* just_via -> just_on

* via -> on and add get_scheduler just_on and on

* get_scheduler calls more widely. Tweak when_all behaviour and mention cancellation.

* Remove get_scheduler, add that in P1898.. Add share.

* Share bullet

* Votes from Prague.

* Let

* Let correction

* Reword handle_error to be more like let

* Remove synchronization primitive wording

* R13 compatibility

* Merge in on changes from P1898R0

* Responses to Eric's feedback down to execution::on

* Eric's feedback applied to when_all

* Tidy handle_error and transform similarly

* Let

* Share

* References and update final

* Eric's minor tweaks from review

* Remove get_scheduler from this paper

* Further changes

* Clarify polls

* Feedback largely from Lewis. Change sync_wait wording to be in terms of std::optional.

* Make sync_wait return types consistent

* Some more of Lewis's feedback

* Lifetime fixes for let and handle_error

* let->let_value handle_error->let_error for consistency

* Tweak wording for operation-state lifetimes.

* Switch sync_wait back to returning T and terminating on done

* Tweak when_all to use single typed senders

* Sync_wait return type. Invocability constraints.

* Fix duplication in let_error definition

* Move let_value before let_error for cleanliness

* Rename share to ensure_started. Various tidying.

Author: LeeHowes