v0.3

• Implement swappable for variant
• Refactor operator == so that the implementation is symmetric
• Refactor safely_constructible trait, and document the numerous
  customization points that now exist
• Add emplace-by-index method, fix corner case behavior of emplace
• Consolidate private visitors of variant class template
• Refactor implementation of apply_visitor, make it noexcept correct,
  also add C++17 visit method syntax
• Remove some cruft from recursive_wrapper