Taocpp Json Versions Save

This version contains a lot of small fixes and improvements as well as a new way to handle initializer lists via the Events interface.

• The PEGTL is now a submodule (previously there was a copy of the PEGTL configured into namespace tao::json::pegtl).
• Countless, mostly small, cleanups, extensions and modernisations to the code as well as adding some missing parts.

• Require C++17.
• Based internal structure on std::variant.
• Based binary views on std::span (when available) or tao::span (fallback).
• Finished JAXN parts parser.
• Fixed compilation with MSVC.
• Switched CI system to GitHub Actions.
• Updated embedded taocpp/PEGTL to version 3.1.1.
• Many small improvements.
• Many small changes.
• Many small fixes.

• Completed/fixed support for the binary formats CBOR, MsgPack and UBJSON.
• Consistent use of traits for construction and comparison.
• Removed support for storing a nullptr in a type::RAW_PTR node.
• Added traits for smart pointers and (some) STL containers.
• Added support for type::OPAQUE_PTR and to generate events from custom values directly.
• Added optional value annotations via a base class.
• Reorganized some files/namespaces for binary formats, JAXN.
• Updated embedded taocpp/PEGTL to version 2.4.0.
• Created some initial documentation.

• Fixed reversed logic on optional getter.
• Fixed compile problem with pre-release versions of Clang 6.
• Updated embedded taocpp/PEGTL to version 2.3.3.
• Enabled clang-tidy checks in TravisCI build.

• Added support for Visual Studio 2017.
• Added tao::string_view (which is an alias to std::string_view when available).
• Added tao::binary_view (which is std::vector<tao::byte>, where tao::byte is an alias to std::byte when available).
• Added support to store tao::string_view or tao::binary_view.
• Routed comparison with direct values through the traits class template.
• Added decoder for the experimental support for UBJSON.
• Fixed and improved small details all over the place.

• Add support for JAXN, a standard for "relaxed JSON".
• Add support for binary data.
• Add support for CBOR / RFC7049.
• Add experimental support for Message Pack.
• Add experimental support for UBJSON (encode only).

• Update embedded taocpp/PEGTL to version 2.0.0.
• Use new lazy tracking mode from the PEGTL.
• Improve parsing efficiency.

• Add experimental support for JSON Reference
• Add experimental support for JSON Schema
• Add type/state DISCARDED to catch errors
• Fix memory leak in some cases if an exception is thrown from a constructor
• Rename a few enums, methods
• Performance improvements

Add SAX consumer and producer for nlohmann/json values. This shows how adaptable and composeable the library is, combining its own powerful parsing, stringify and prettify capabilities with third-party value classes.