Fix just_from sink return type

[fallintoplace]

What
Return completion signatures from the just_from sink.

Why
Callbacks using auto completions = sink(42); currently fail to compile.

Implementation
Make the runtime sink return the same type as the probe. This also fixes just_error_from and just_stopped_from.

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[RobertLeahy]

What's the use case for auto completions = sink(42);?

Since invoking sink ends the operation performing actions thereafter should, in general, be considered an anti-pattern imo, which makes me think that disallowing auto completions = sink(42); is a feature.

[fallintoplace]

Fair point, I agree that doing work after completion should be avoided. The intended use case is actually the documented return sink(42, 3.14); example in include/exec/just_from.hpp. The probe sink returns completion_signatures, but the runtime sink returns void, so the documented pattern fails to compile when the operation is started. This change makes the two consistent without requiring additional work after completion.