Init `sbox` within `xor_hasher`

[Rudxain]

That vector is always initialized to all 0s before being passed to that fn, so it makes sense to create it there and return it after mutating, instead of passing a mutable reference and manually resetting with fill

[Rudxain]

I think this was a mistake. This could worsen performance when the file list is larger than 1.

Related to #16

Link to the commit that closed this as "Not Planned": https://github.com/Rudxain/xorsum/commit/b0487046a4b503fe05088f507039c37c1281bde1

[Rudxain]

I wish xor_hasher was a pure fn, but this is not good for the heap