scripted-diff: Use std::span over Span

-BEGIN VERIFY SCRIPT-

 ren() { sed -i "s!\<$1\>!$2!g" $( git grep -l "$1" -- "./src" ":(exclude)src/span.h" ":(exclude)src/leveldb/db/log_test.cc" ) ; }

 ren Span            std::span
 ren AsBytes         std::as_bytes
 ren AsWritableBytes std::as_writable_bytes

 sed -i 's!SpanPopBack(Span!SpanPopBack(std::span!g' ./src/span.h

-END VERIFY SCRIPT-

src/bench/load_external.cpp

@@ -46,7 +46,7 @@ static void LoadExternalBlockFile(benchmark::Bench& bench)
     ss << static_cast<uint32_t>(benchmark::data::block413567.size());
     // We can't use the streaming serialization (ss << benchmark::data::block413567)
     // because that first writes a compact size.
-    ss << Span{benchmark::data::block413567};
+    ss << std::span{benchmark::data::block413567};
 
     // Create the test file.
     {