conditionally contribute set_error_t(std::exception_ptr) in repeat_* senders

[justend29]

Currently, the repeat_ family of sender adapters invariantly contributes set_error_t(std::exception_ptr) to its completion signatures, which I first mentioned in this issue. This PR causes repeat_until, repeat, and repeat_n to conditionally contribute set_error_t(std::exception_ptr) to the completion signatures. Each of the possible cases (not) to contribute the completion signature are individually tested.

It also contains some small corrections that were originally in #1797, until @RobertLeahy informed me that it is possible to detect the noexcept-ness of a connect using STDEXEC::__receiver_archetype<Env>, wherein I returned to this original goal.

This is also my first time working with the meta-programming system in this library, so feedback is welcome.

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[justend29]

Thank you for the review, @ericniebler. All of your comments have been addressed, though __partition_completion_signatures_t was changed to __partitions_of_t to compile.

[ericniebler]

/ok to test ac8352c