Add SipHash Family of Keyed PRFs

[NotsoanoNimus]

Implementation of the SipHash family of pseudorandom functions, complete with accompanying tests.

Predefined modules include:

• siphash24 - Performant - 64-bit hash from 2 rounds-per-block and 4 finalization rounds.
• siphash24_128 - Same as above but for 128-bit outputs.
• siphash48 - Hardened - 64-bit hash from 4 rounds-per-block and 8 finalization rounds.
• siphash48_128 - Same as above but for 128-bit outputs.

Out-of-the-box usage:

ulong hash = siphash24::hash(input, runtime_key);

for (usz i = 0; i < 4; ++i)
{
    // overloaded '[]' operator sets Bloom filter bits accordingly
    bloom_filter[(hash >> (16 * i)) & 0xFFFF] = true;
}

Other non-standard variations of SipHash can be used with an alias declaration like so:

import std::hash::siphash;

alias MySipHashFlavor = SipHash { uint128, 8, 16 };
alias hash = siphash::hash { uint128, 8, 16 };

[lerno]

I posted some suggested improvements above.

[NotsoanoNimus]

I incorporated your feedback, and then some. I tend to put a lot of idiosyncrasies around explicit casts and parentheses in my code because I'm paranoid of C's UB and other spooky behaviors, so it'll take a bit for me to be a little more lax with the help of C3.

Thank you for your time reviewing this.

[lerno]

Updated the test to use this:

char[64] plaintext = math::iota(char[64]);