Add test for target_include_directories

Tag binary values in cbor if set

Clean up maintainer Makefiles and fix some linter warnings

Add option to ignore CBOR tags

Hash function without allocation

Add ClangCL for MSVC

MS Visual Studio 2019 has builtin support for other compiler toolsets [1].

This commit adds support for compiling using LLVM/Clang 10 using Visual
Studio.

ClangCL pretends to be MSVC so the usual MSVC flags apply, see also [2].
For detecting if ClangCL is used, newer cmake verisons (>= 3.15) have
builtin support using CMAKE_CXX_COMPILER_FRONTEND_VARIANT [3], for older
ones a workaround is available using CMAKE_CXX_SIMULATE_ID [4].

[1]: https://devblogs.microsoft.com/cppblog/clang-llvm-support-in-visual-studio/
[2]: https://clang.llvm.org/docs/UsersManual.html#clang-cl
[3]: https://stackoverflow.com/a/10055571
[4]: https://github.com/microsoft/CMake/commit/4fe34b2d29ca67ea91b2545b223dad8abbcad282

Makefile: Always use SED variable

Add Xcode 12 CI

We need to have a proper sed, even on MacOSX. So let's use the variable
introduced in 191aa0fd (🔧 overworked maintaner targets,
2019-03-28) in more places.

Add ordered_json specialization with ordered object keys

Make library work with Clang on Windows

 Conflicts:
	include/nlohmann/detail/input/binary_reader.hpp
	include/nlohmann/detail/input/json_sax.hpp
	include/nlohmann/detail/input/lexer.hpp
	include/nlohmann/detail/input/parser.hpp
	include/nlohmann/detail/json_pointer.hpp
	include/nlohmann/detail/output/serializer.hpp
	include/nlohmann/json.hpp
	single_include/nlohmann/json.hpp