Make sync_wait a proper CPO.

[egorich239]

The sync_wait CPO by default defers to sync_wait_r</*deduced-type*/> which matches the preexisting behavior. Overriding sync_wait_r<T> hence by default is enough to consistently customize the behavior of both algorithms.

As an aside I matched piped API of sync_wait and sync_wait_r.

[AnujYamdagni]

We're doing repo cleanup - Can you add more information in your summary about the motivation behind this PR?

[LeeHowes]

The POR is that we don't want pipe syntax for sync_wait, so that aspect of this we shouldn't merge.

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