New macros for the named JSON convertor generation

[radistmorse]

This patch adds several new NLOHMANN_DEFINE_<***>_WITH_NAMES macros. They behave the same way as the ones without WITH_NAMES, but require explicit JSON names. Useful for the situation when the fields in the class are following the naming convention that you do not want to expose to JSON, e.g.

    class address {
      private:
        std::string m_street;
        int m_housenumber;
        int m_postcode;

      public:
        NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(address, "street", m_street, "housenumber", m_housenumber, "postcode", m_postcode)
    };

or if the name in JSON cannot be used as the field name because it is reserved, e.g.

    struct weird {
        std::string static_comp;
        std::string class_type;
        std::string public_field;
    };
    NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES(weird, "static", static_comp, "class", class_type, "public", public_field)

Also, this patch includes the unit tests for the new macros, and the update for the documentation.

It also fixes a small error in the docs where the DEFINE_TYPE macros were said to support up to 64 member variables, when in reality it is 63.

This is the update to the #4092 which didn't pass the amalgamation test because I had an old version of astyle. I updated the patch to include the new DERIVED variations as well as ONLY_SERIALIZE. Hopefully this one will be better.

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Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

coverage: 99.639%. remained the same
when pulling 27b9408 on radistmorse:develop
into af4ad79 on nlohmann:develop.