using more CMake

[nlohmann]

Done:

• using CMake for the special targets Valgrind (JSON_Valgrind), Clang sanitizers (JSON_Sanitizer), and no-exceptions (JSON_NoExceptions)
• using Ninja to improve compilation speed
• compiling and executing each test suite individually
• removed test cases that broke the MSVC 2017 build

Missing:

• some magic to get Coveralls to work with individual executables (maybe @theodelrieu can help here with his approach from Refactor/split it #700)

[nlohmann]

(This should fix #698)

[coveralls]

Coverage remained the same at 100.0% when pulling e2045ea on feature/issue698 into 7435d54 on develop.

[nlohmann]

@theodelrieu Would it be OK if I merged this PR tonight?

[theodelrieu]

Sure! Even now if you want