Adapt o3tl::span to updated C++2a std::span

...where index_type has changed from ptrdiff_t to size_t, see <https://
github.com/cplusplus/draft/commit/7f2493e2e2d34b42a6c12c8806e536d4feb3aee3>
"P1227R2 Signed ssize() functions, unsigned size() functions".  Ideally,
ece27693ba "Adapt to C++2a std::span<>::index_type
being unsigned size_t" could be simplified now to use std::size_t, but there may
be build environments that include a C++2a <span> that hasn't been adapted to
P1227R2 yet.

Change-Id: I7bfc0da65d415ef06e94d95b5f62030aeb8b35f6
Reviewed-on: https://gerrit.libreoffice.org/69391
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/o3tl/span.hxx

@@ -40,7 +40,7 @@ public:
     using iterator = pointer;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
     using reverse_iterator = std::reverse_iterator<iterator>;
-    using index_type = std::ptrdiff_t;
+    using index_type = std::size_t;
     using difference_type = std::ptrdiff_t;
 
     constexpr span() noexcept : data_(nullptr), size_(0) {}
@@ -52,7 +52,7 @@ public:
         : data_(a), size_(len)
     {
         // not terribly sure about this, might need to strengthen it
-        assert((a == nullptr && len == 0) || (a != nullptr && len >= 0));
+        assert(a != nullptr || len == 0);
     }
 
     constexpr bool empty() const noexcept { return size_ == 0; }
@@ -75,7 +75,7 @@ public:
     constexpr index_type size() const noexcept { return size_; }
 
     constexpr reference operator [](index_type pos) const {
-        assert(0 <= pos && pos < size());
+        assert(pos < size());
         return data_[pos];
     }