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

arena.h

@@ -1,6 +1,4 @@
-#ifndef BASE_ARENA_H
-#define BASE_ARENA_H
-
+#pragma once
 #include "base.h"
 
 // #define _CRT_SECURE_NO_WARNINGS
@@ -425,5 +423,3 @@ void *plat_mem_reserve(u64 size);
 b32 plat_mem_commit(void *ptr, u64 size);
 b32 plat_mem_decommit(void *ptr, u64 size);
 b32 plat_mem_release(void *ptr, u64 size);
-
-#endif // BASE_ARENA_H