Allow to use string_view from std::experimental

[karzhenkov]

The support of string_view was not fully tested due to problems discussed in #2213, #2241, #2117. When using clang-7 in C++17 mode the string_view is defined in the std::experimental namespace. This results in a compile error after travis configuration is corrected.

[coveralls]

Coverage remained the same at 100.0% when pulling 5f566f0 on karzhenkov:fix-ts-string_view into b9d7651 on nlohmann:develop.

[karzhenkov]

It would be better to specify CXX_STANDARD=20, however, cmake v3.9.2 at Travis doesn't support such option. Anyway, this setting had no effect before.

[karzhenkov]

This introduces a mix of qualified and unqualified names in source code. E.g. we have std::string but string_view instead of std::string_view. We cannot introduce any names (such as std::expermental::string_view) directly into std. There is other option to consider:

namespace nlohmann {
    namespace std_aliases {
        using namespace std;
    }
}

This allows regular qualification approach (std_aliases::string and std_aliases::string_view) but requires a lots of changes in almost all files.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.