Remove static asserts

[theodelrieu]

This PR addresses issues #960.

There is some refactoring in detail/meta, the checks we perform were too brittle (I fixed some issues in the mean time).

At some point I'd like to iron out the quirks in meta, (e.g. is_compatible_* traits, etc...), but this is not the goal of this PR.

I will sum up the pros/cons of this PR that can be found in detail in #960:

Pros:

• nlohmann::json is now more SFINAE-correct. The library has better interoperability with other generic libraries. (Not 100% because of operator T() considered harmful #958 though)
• No API breakage
• Correct fix for Fix operator== overload ambiguity #1067 Compilation error when using MOCK_METHOD1 from GMock and nlohmann::json #1212, from_json declaration order and exceptions #561

Cons:

• No more static_assert, instead the compiler will yell that no constructor could be used.

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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 file single_include/nlohmann/json.hpp. The whole process is described here.

[coveralls]

Coverage remained the same at 100.0% when pulling aea648b on theodelrieu:remove_static_asserts into eb30ff0 on nlohmann:develop.

[nlohmann]

To me, the PR is SFIENA magic... :-S

@gregmarr Do you think the PR is OK to merge?

(I do trust @theodelrieu, but I really fail to understand the individual changes...)

[gregmarr]

Those things that I can follow look good. I guess as long as the test suite is still passing, we should be good.

[nlohmann]

Thanks a lot!