STEllAR-GROUP / hpx / #633

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Merge #5674

5674: Allow using  `any_sender` in global variables r=msimberg a=msimberg

## Proposed Changes

This allows storing `(unique_)any_sender` in global variables (or even function-local statics). The problem is that the destructor of the `any_sender` may hold e.g. hold an `any_operation_state/receiver` from a type-erased predecessor sender. When the global `any_sender` is destroyed it may try to access an empty vtable that was destroyed before the global `any_sender` was destroyed. The failure case in the added test is a segfault. The `empty` method on the (supposed) empty vtable happens to return `false` and the destructor tries to delete the empty vtable even though it was never dynamically allocated.

~This changes the empty vtables to be `inline constexpr` variables.~ This adds a helper base class which accesses the `any_operation_state` and `any_receiver` empty vtables when constructing `(unique_)any_sender`.

Flyby:
- Remove `explicit` from the `(unique_)any_sender` constructors. It's very convenient to be able to simply assign any sender to a `(unique_)any_sender` without spelling out the type.

## Checklist

Not all points below apply to all pull requests.

- [ ] I have added a new feature and have added tests to go along with it.
- [x] I have fixed a bug and have added a regression test.
- [ ] I have added a test using random numbers; I have made sure it uses a seed, and that random numbers generated are valid inputs for the tests.


Co-authored-by: Mikael Simberg <mikael.simberg@iki.fi>

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