Apply changes from P3187R1

[lewissbaker]

Removes the following facilities as specified in P3187R1:

• ensure_started()
• execute()
• start_detached()

Also removes the following facility as it does not make sense any more:

• execute_may_block_caller

Note: ensure_started() algorithm had wording shared with split() The common wording has now been simplified to now only describe split().

[lewissbaker]

for thoroughness we should also remove mentions of start_detached, execute, and ensure_started from other parts of the paper.

I wasn't sure whether to leave them there but just add notes that they are no longer in the design or just remove them wholesale.