Fixed compiler error in VS 2015 for debug mode

https://github.com/nlohmann/json/issues/1114

fixed compile error for #1045

Fix links to cppreference named requirements (formerly concepts)

meson: fix include directory

"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.

Added support for string_view in C++17

The branch coverage reported by lcov is weird. The code before and after has the same Godbolt assembler, but the code with the lambda has a better branch coverage.

 test (non)equality for alt_string implementation 

Feature/unordered map conversion

Fixes #1133