"Concepts" have been renamed to "named requirements".
This is because P0898R3 Standard Library Concepts has been merged into
C++20.
Cppreference have moved their links accordingly.
* Rename 'develop' folder to 'include/nlohmann'
* Rename 'src' folder to 'single_include/nlohmann'
* Use <nlohmann/*> headers in sources and tests
* Change amalgamate config file