Seed SipHash with 128-bit key
Seed SipHash with 128-bit key

SipHash expects a 128-bit key, and we were indeed generating 128-bits,
but restricting them to hex characters 0-9a-f, effectively giving us
only 4 bits-per-byte of key material, and 64 bits overall.

Now, we skip the hex conversion and supply 128 bits of unfiltered
random data.

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