Add helper types to make it easier to create a basic_json type with modified template parameters

[barcode]

(First part of the points listed in #3110 (comment), the rest is done in a different PR to prevent mixing of separate issues / features)

Make specifying a base class easier/less verbose.

This PR adds member templates to nlohmann::json which can be used to create a basic_json type with one replaced template parameter (e.g. nlohmann::json::with_changed_json_serializer_t<T>).

Tests and examples were modified to use those templates (except for regression tests).

Open questions from my side:

• Should we add something like this? (I think so, but what is your opinion?)
• Should we go with the current names (with_changed_*) or rather use something else (e.g. with_*)?

In case this will be added, i will write the documentation.

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Pull request checklist

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• 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: 100.0%. remained the same when pulling 0d9f46f on barcode:make_specifying_adapted_json_type_easier into 5d27543 on nlohmann:develop.

[barcode]

Work on this PR is done and it only needs a review.