v0.4.0

• Fixed soagen::is_table<>
• Added support for emplace-constructing column values by unpacking all std::tuple-like types (not just the emplacer)
• Added support for taking std::integral_constants in for_each_column()
• Added soagen::same_table_type<>
• Added conversions between soagen::row<> specializations
• Optimized instantiation overhead for most type-traits