optimisation: implement full s-box precomputation

Compute 32-bit rotated words in init_sbox_x(); in f() only xor them.

Benchmark with 80MiB file:

Before:
real    0m4.230s
user    0m4.038s
sys     0m0.184s

After:
real    0m1.579s
user    0m1.395s
sys     0m0.180s