All executor facilities should be `constexpr` when possible

[miscco]

Currently we have a some facilities, e.g. some adaptors and most factories that are not specified to be constexpr

I believe we should add constexpr support whenever possible. Yes it will not help at runtime, looking at recent improvements for compile time testing of user code, we should not restrict ourselves needlessly.

If there is no significant opposition coming up I will prepare a PR to fix that when possible

[miscco]

As we are starting an independent implementation, I would really like to get an opinion here.

I still do not see why we should make any of those facilities not constexpr

Of course they wont be used in a constexpr context most of the time but that is no argument in preventing it.