Simplify basic action implementations

[K-ballo]

No description provided.

[K-ballo]

Why is this needed? and is it the correct way to declare such an action?

[hkaiser]

Very nice, thanks!

[sithhell]

clang doesn't like all of your changes:

/hpx/source/hpx/runtime/actions/component_action.hpp:91:5: error: 'action' defined as a class template here but previously declared as a struct template [-Werror,-Wmismatched-tags]
    class action<R (Component::*)(Ps...), F, Derived>
    ^
/hpx/source/hpx/runtime/actions/basic_action.hpp:528:5: note: did you mean class here?
    struct action;
    ^~~~~~
    class

[hkaiser]

There are also additional test failures caused by the proposed changes: https://stellar-group.gitlab.io/-/hpx/-/jobs/159443858/artifacts/tests.unit.html#775cc3d3-3e99-462e-91fa-e374cdeb7c28

[hkaiser]

@sithhell, @K-ballo: were the test failures referenced above fixed now?

[K-ballo]

@K-ballo: were the test failures referenced above fixed now?

Yes, by that commit after the reference. A const-qualified component type was sometimes used to do the pinning whereas a mutable component is required, and since pinning is SFINAE-based it would simply ignore the request instead of diagnosing the issue.