Revert "rust: Refactor PMAC XOR ordering"

[tarcieri]

This reverts commit a3c96e5.

This commit caused a pretty severe performance regression. Below are the numbers with the commit reverted:

test bench::bench_aes_pmac_128_mac_1024_bytes          ... bench:         544 ns/iter (+/- 114) = 1882 MB/s
test bench::bench_aes_pmac_128_mac_128_bytes           ... bench:          99 ns/iter (+/- 17) = 1292 MB/s
test bench::bench_aes_pmac_128_mac_16384_bytes         ... bench:       8,319 ns/iter (+/- 1,451) = 1969 MB/s
test bench::bench_aes_pmac_siv_128_encrypt_1024_bytes  ... bench:       1,049 ns/iter (+/- 185) = 976 MB/s
test bench::bench_aes_pmac_siv_128_encrypt_128_bytes   ... bench:         387 ns/iter (+/- 91) = 330 MB/s
test bench::bench_aes_pmac_siv_128_encrypt_16384_bytes ... bench:      12,946 ns/iter (+/- 5,471) = 1265 MB/s

Here they are prior to the revert:

test bench::bench_aes_pmac_128_mac_1024_bytes          ... bench:       1,017 ns/iter (+/- 146) = 1006 MB/s
test bench::bench_aes_pmac_128_mac_128_bytes           ... bench:         155 ns/iter (+/- 29) = 825 MB/s
test bench::bench_aes_pmac_128_mac_16384_bytes         ... bench:      16,116 ns/iter (+/- 3,860) = 1016 MB/s
test bench::bench_aes_pmac_siv_128_encrypt_1024_bytes  ... bench:       1,404 ns/iter (+/- 447) = 729 MB/s
test bench::bench_aes_pmac_siv_128_encrypt_128_bytes   ... bench:         291 ns/iter (+/- 56) = 439 MB/s
test bench::bench_aes_pmac_siv_128_encrypt_16384_bytes ... bench:      19,427 ns/iter (+/- 3,715) = 843 MB/s

The original version is a full 50% faster.