Add span::empty() and fix const-correctness bug in as_writable_bytes()

include/unifex/span.hpp

@@ -93,6 +93,10 @@ struct span {
  return Extent;
  }
 
+ constexpr bool empty() const noexcept {
+ return Extent == 0;
+ }
+
  T* begin() const noexcept {
  return data();
  }
@@ -173,6 +177,10 @@ struct span<T, dynamic_extent> {
  return data_;
  }
 
+ bool empty() const noexcept {
+ return size_ == 0;
+ }
+
  std::size_t size() const noexcept {
  return size_;
  }
@@ -294,7 +302,7 @@ span<std::byte, Extent * sizeof(T)> as_writable_bytes(
  const span<T, Extent>& s) noexcept {
  constexpr std::size_t maxSize = std::size_t(-1) / sizeof(T);
  static_assert(Extent <= maxSize);
- return span<const std::byte, Extent * sizeof(T)>{
+ return span<std::byte, Extent * sizeof(T)>{
  reinterpret_cast<std::byte*>(s.data())};
 }