Modernize code base for C++17

[dominiklohmann]

• The overload_set utility from the original library is quite outdated and can be replaced with a simper construct.
• The get_type_name utility can be optimized not to require RTTI as a fallback.
• The library makes unnecessary copies in lots of lambdas.
• Some functions can be moved into source files to avoid ODR violations.
• Generally, the library can be split into multiple files. wahl/wahl.hpp should include all.