Replace evergreen-tree byte encoding with Unicode encoding to prevent VS warnings

[FelixPetriconi]

The latest Visual Studio compiler raises a warning in our client code: sol2\include\sol\inheritance.hpp(56,29): warning C5321: nonstandard extension used \'xF0\' as a multi-bute utf-8 character. Use \u instead for cross platform compatibility or \'Zc:u8EscapeEncoding\' to disable the extension.

I raised here an issue at MS. At that time adding the recommended compiler switch was the easier solution. But we are now facing the problem in more and more products who use the Sol library and so I decided to create this PR.

I noticed during my fix that the base_class_index_propagation_key() function and the one below are nowhere referenced in the whole repo. Purpose or accident?