- basic functionality similar to std::span (C++20).
  Holds pointer and size: for lightweight handling of address ranges.

- implements cdata_bytes() and data_bytes() methods for similarity
  with UList. For span, however, both container accesses are const
  but the data_bytes() method is only available when the
  underlying pointer is non-const.

  No specializations of std::as_bytes() or std::as_writeable_bytes()
  as free functions, since std::byte etc are not available anyhow.