refactor: Avoid passing span iterators when data pointers are expected

For Span, iterators are just raw data pointers. However, for std::span
they are not.

This change makes it explicit where data pointers are expected.

Otherwise, there could be a compile error later on:

  No known conversion from 'iterator' (aka '__normal_iterator<const std::byte *, std::span<const std::byte, 18446744073709551615>>') to 'std::byte *'.

src/streams.h

@@ -79,7 +79,7 @@ public:
             memcpy(vchData.data() + nPos, src.data(), nOverwrite);
         }
         if (nOverwrite < src.size()) {
-            vchData.insert(vchData.end(), UCharCast(src.data()) + nOverwrite, UCharCast(src.end()));
+            vchData.insert(vchData.end(), UCharCast(src.data()) + nOverwrite, UCharCast(src.data() + src.size()));
         }
         nPos += src.size();
     }