Error in AppVeyor:

unit-conversions.cpp(441): error C2440: 'initializing': cannot convert from 'nlohmann::json' to 'std::nullptr_t'

(https://ci.appveyor.com/project/nlohmann/json/build/2838/job/wo7im01sq6tvhe9m)

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