The element type of flux::cartesian_product, cartesian_power and adjacent is a tuple of
 the element type(s) of the underlying sequences. These three adaptors also provide
  *_map versions, which take an n-ary function and call it directly with the underlying
  elements. This is to avoid forming a tuple and then immediately destructuring it, as
  would happen in a subsequent call to map(unpack(func)).

 This commit adds the *_map version for the zip adaptor

flux::empty<T> now supports any T, while the data() function requires that T satisfies std::is_object_v<T>

… and 0 sequences