Apply changes from P3187R1 (#254)

* Remove `ensure_started` algorithm

Applying changes from paper P3187.

Changes were more complicated due to the merging of split() and
ensure_started() wording. This wording has now been simplified to
now only describe the split() algorithm.

* Remove start_detached() and execute() functions.

Applies changes from P3187R1.

Note that this change also removes the `execute_may_block_caller`
query as this was only relevant to the `execute()` function which
is also removed here.

* bug fixes

* Update execution.bs

* remove other mentions of `ensure_started`, `start_detached` and `execute`

* fix bug in `split`'s `start` operation that lewis spotted

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Co-authored-by: Lewis Baker <[email protected]>
Co-authored-by: Eric Niebler <[email protected]>