optional

What's Changed

• Fix implementation of optional<T&>::emplace() in https://github.com/TartanLlama/optional/pull/34
• Fix mistakes in README.md in https://github.com/TartanLlama/optional/pull/35
• Revamp CMake files to be more understandable and correct in https://github.com/TartanLlama/optional/pull/36
• Added explicit default initialization in copy constructor in https://github.com/Tartan...

• Add install targets for CMake
• Fixes to swap implementation

• Bugfixes for optional references
• Constexpr fixes
• Play nice with CMake add_subdirectory

• Support mapping functions which return references
• Support tl::optional<T&>

• Fix assignment chaining
• More testing
• Compatibility with https://github.com/TartanLlama/expected