C++ overload resolution/list initialization rules are hard.

std::initializer_list does not own the temporaries created in
its initialization. Therefore, storing it in an independent
stack variable is unsafe.

This commit works around an issue in std::initializer_list design.
By using a detail::json_ref proxy with a mutable value inside,
rvalue-ness of an input to list initializer is remembered and
used later to move from the proxy instead of copying.

We now rely on implicit conversions to an input_adapter object in the parse/accept functions.

Travis found an error with Clang 3.8's sanitizers, see https://travis-ci.org/nlohmann/json/jobs/256366699. Unfortunately, I cannot reproduce this error with clang version 6.0.0 (trunk 308825) locally. However, this seems to be an issue, because so far, we did not reset a value after moving from it.

Internally, the parser now writes its result into a JSON value provided as a reference. To be usable, the public interfaces need to be extended.

- Replaced codepoint calculation by bit operations.
- Fixed several clang-tidy warnings.

A complete rewrite of the string escape function. It now provides codepoint-to-\uxxxx escaping. Invalid UTF-8 byte sequences are not escaped, but copied as-is. I haven’t spent much time optimizing the code - but the library now agrees with Python on every single Unicode character’s escaping (see file test/data/json_nlohmann_tests/all_unicode_ascii.json).

Other minor changes: replaced "size_t" by "std::size_t"

Rename BuildTests to JSON_BuildTests

added Contributor Covenant Code of Conduct

add ensure_ascii parameter to dump. #330

This avoids potential conflicts with other libraries when the library is
built from source and included in a bigger cmake build.

Refacto/split basic json

Those macros are used to reduce template argument boilerplate

These versions are still not working properly: https://travis-ci.org/nlohmann/json/jobs/251675281

As <iostream> is not included in json.hpp any more, all code examples need to include <iostream> now.

Don't include <iostream>, use std::make_shared

Maybe the simple approach is working now - at least it is in https://github.com/taocpp/PEGTL/blob/master/.travis.yml

avoid bloat caused by <iostream> and std::cout and friends in apps where
iostream are not used.