Fixes yaito3014/rvariant#39

• Remove eagerly evaluated mutual dependency between operator<=> and other relational operators in rvariant
• Fix mutual dependency between indirect and recursive_wrapper
  • MSVC somehow considers both classes' relational operators during overload resolution, resulting in circular dependency. I'm not sure if MSVC's behavior is standard-conformant or not, but we do need a workaround. This is due to the class inheritance, so we just move the actual implementation to detail::indirect_base as a workaround.
• Fix a bug in synth_three_way where it used wrong concept
• Some refactoring