perl 5.18改用siphash

siphash 把H(m) mod l 的H也搞成伪随机，碰撞的复杂度从O(n)变成O(n^2)

见：
https://metacpan.org/module/RJBS/perl-5.18.0/pod/perldelta.pod
https://131002.net/siphash/
https://131002.net/siphash/siphash.pdf