Simplify SOAExtensions

• add deduction guide for ZipContainer
• use template deduction when possible
• prefer defaulted {copy,move,}constructors
• prefer binary operators implemented as friends
• prefer fold expression over recursion
• prefer to pass sink arguments by value, and move them
• add an overload to semantic_zip which does not require the skin to be specified, and use this overload to simplify TestSelection
• rename always{True,False} to always{True,False}_t, and add two inline constexpr variables always{True,False} so callers do not have to add {}