API: clean up and modernize value types

Value types should

• be movable (ideally with both move-constructor and move-assignment operator)
• use explicit constructors from other types
• use noexcept as appropriate (for all implicitly shared classes: moving, copying, swapping, comparing)
• not have any protected or virtual members (subclassing a value type is not allowed)
• use hidden friends for operators

Types can either have none of the special member functions defined, or all of them.