Add serialization-only user defined type macros

[ZeronSix]

This pull requests adds NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE and NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE macros.

Suppose we have a non-default-constructible type person and another type that contains a field of type person. For example:

#include <nlohmann/json.hpp>

#include <string>
#include <vector>

struct person {
    std::string name;
    int age;

    person(std::string name, int age)
        : name{std::move(name)}
        , age{age}
    {}
};

struct persons {
    std::vector<person> list;
};

NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, age);
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(persons, list);

An attempt to use the NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE macro to generate serialization/deserialization code will lead to a compilation error:

test.cpp: In function ‘void from_json(const json&, persons&)’:
include/nlohmann/detail/macro_scope.hpp:385:81: error: no matching function for call to ‘nlohmann::json_abi_v3_11_2::basic_json<>::get_to(std::vector<person>&) const’
  385 | #define NLOHMANN_JSON_FROM(v1) nlohmann_json_j.at(#v1).get_to(nlohmann_json_t.v1);

README.md provides an example how to fix this by providing a specialization of adl_serializer. However, this requires writing quite a bit of code and might even be unnecessary in cases when we only need serialization without deserialization.

In this situations, we can use added macros to generate only to_json functions.

NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(person, name, age);
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(persons, list);

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Pull request checklist

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.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for this kind of bug). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[coveralls]

coverage: 100.0%. remained the same when pulling ba7f94d on ZeronSix:only-serialize-macros into edffad0 on nlohmann:develop.

[nlohmann]

Please add documentation. See https://github.com/nlohmann/json/blob/develop/docs/mkdocs/docs/api/macros/index.md for a starting point.

[nlohmann]

(Also, please update from the develop branch as it has a working CI now.)

[ZeronSix]

I also noticed that the source code highlighting is a bit off in these pages, see #4169.

[nlohmann]

Looks good to me.