Use spans of std::byte in serialize

This switches .read() and .write() to take spans of bytes.

src/node/blockstorage.cpp

@@ -435,7 +435,7 @@ bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatFilePos& pos, c
         }
 
         block.resize(blk_size); // Zeroing of memory is intentional here
-        filein.read((char*)block.data(), blk_size);
+        filein.read(MakeWritableByteSpan(block));
     } catch (const std::exception& e) {
         return error("%s: Read from block file failed: %s for %s", __func__, e.what(), pos.ToString());
     }