Enable compile-time evaluation of just senders

[romintomasetti]

Motivation

C++23's expanded constexpr support enables compile-time computations with complex types, but stdexec currently lacks the annotations needed to use sender/receiver patterns in constant expressions.

Changes

This PR adds constexpr annotations to make just constructible and connectable at compile time.

See the added test for usage.

I've tried to make then work too, so that at least some interesting sender can be constructed that really does some work, but there were too many issues I couldn't fix myself.

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[romintomasetti]

@ericniebler It might be a first step towards yours (#1772).

[RobertLeahy]

Rationale for closing #1771 seems to apply here as well.