...see <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0122r7.pdf>
"span: bounds-safe views for sequences of objects".  o3tl::span is still an
incomplete approximation of std::span; removed those o3tl::array_view members
that are not present in std::span (and were not used in the code).

Relies on C++17 __has_include to use standard <span> where available (e.g., in
LLVM 7 libc++).

Change-Id: I82a7e246b61b2456fa6183025d25eec4121ad3c9
Reviewed-on: https://gerrit.libreoffice.org/66215
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>