C++20: Replace all SFINAE with Concepts

[johannesugb]

Concepts are a new feature introduced into C++20. Make yourself familiar with the topic. You can get an overview of it in the following video:

• C++20: An (Almost) Complete Overview - Marc Gregoire - CppCon 2020 starting at 18:57

At many places in the framework, SFINAE are used which basically serve the same purpose, but are more complicated to use. The SFINAE classes are the following:

• class is_dereferenceable
• class has_resize
• class has_size_and_iterators
• class has_nested_value_types

Definition of done:

• Search for all usages of the SFINAE classes mentioned above and search for all usages of std::enable_if. Replace them with Concepts => i.e. use the newly introduced C++20 requires keyword.
• Delete all the SFINAE classes mentioned above from the codebase.
• For each refactored function/method, make sure it is properly documented and the Contribution Guidelines have been followed.