Move C++20 things into `std` namespace

[marehr]

In Core Meeting (2020-03-23) we decided that stuff that will be part of C++20 and that we implement in seqan3 already should be defined in the std namespace and included via the respective header.

This means

• [x] move remove_cvref_t to std/type_traits. https://github.com/seqan/seqan3/blob/7a360e2e3bc32c9264363aa106784359aa63a834/include/seqan3/core/type_traits/basic.hpp#L35
• [x] Issue seqan/seqan3#1662 was part of this

Take seqan/seqan3#1681 as a basis, but don't put remove_cvref_t in seqan3::cpp20, but directly in the std namespace.

?? I find no more examples that are c++20, but defined in seqan3; Some standard concepts ??

[marehr]

@wvandertoorn, @simonsasse Are you interested in finishing this ticket?

[wvdtoorn]

Yes, I'll work on this

[marehr]

Fixed by seqan/seqan3#2079