hashers now use thread local arena

Instead of writing directly to file_hashes.txt, hash_workers now are
using a local arena, writing everything once at the end

using #pragma once to ensure that a given header file is included only
once in a single compilation unit

binaries/changelog.txt

@@ -20,3 +20,7 @@ Fix bug multiple threads committing at the same time, fixed by using atomic_flag
 Reorder helper functions
 
 v3.4: Rewriting hash_worker() to export file_hashes.txt
+
+v4.0: Instead of writing directly to file_hashes.txt, hash_workers now are using a local arena, writing everything once at the end
+using #pragma once to ensure that a given header file is included only once in a single compilation unit
+forcing xxhash to use the stack instead of the heap