Support Apple's Swift Package Manager

[aleksproger]

Reason

• Main way to develop under Apple's platforms is using SPM

Changes

• Added Package.swift
• Added _SwiftPackageManagerFile.cpp to single_include/nlohmann in order SPM to see it as a C++ source file
• Added .build to .gitignore

Context

• SPM is a package manager for Swift and C-based languages. It is the main way to develop under Apple's platforms. It is also used to build Swift packages for Linux.
• Adding support for SPM will significantly simplify usage of this library for Apple platforms

[Describe your pull request here. Please read the text below the line, and make sure you follow the checklist.]

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• [x] Changes are described in the pull request, or an existing issue is referenced.
• [x] The test suite compiles and runs without error.
• [x] Code coverage is 100%. Test cases can be added by editing the test suite.
• [x] 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 b87c908c01b76d15fe0d501ea733e9e639e9be6c on aleksproger:feature/sapitskiy/spm-support into edffad036d5a93ab5a10f72a7d835eeb0d2948f9 on nlohmann:develop.

[aleksproger]

@nlohmann may you please re-trigger Ubuntu workflows? I see some of those failed, but suppose it should be flakinesses.

[nlohmann]

No, there is an issue with the CI right now.

[aleksproger]

@nlohmann may you trigger this one? As the CI now in a good state.

[nlohmann]

Please update to the latest develop to have a fixed CI.

[aleksproger]

@nlohmann rebased PR, have a look

[aleksproger]

@nlohmann minGW checks have failed. May you retry or this is some persistent failure?

[nlohmann]

It is fixed on the latest develop branch. Please update, it should work now.

[aleksproger]

@nlohmann trigger CI, please

[aleksproger]

@nlohmann trigger CI once again please I had to update the PR, as I noticed deprecation warnings for and bumped the supported platforms for watchOS, tvOS Otherwise it's good to merge and passed all the checks in previous iteration.

[richardtop]

Looking forward to getting this merged!

[nlohmann]

Thanks!