Build is working again (on Zig master branch)

Re-added specifying thread count doing something
Added single-threaded performance to benchmark.sh
Added single-threaded test (currently getting stuck forever)
Various minor fixes revealed now that build is working again.

src/util/offset_file.zig

@@ -18,7 +18,7 @@ pub fn init(io: Io, fil: File, archive_size: u64, init_offset: u64) !OffsetFile
         }),
     };
 }
-pub fn deinit(self: @This(), io: Io) void {
+pub fn deinit(self: *OffsetFile, io: Io) void {
     self.map.destroy(io);
 }