Use siphash for persistent hashing

[inducer]

I don't know how much of a bottleneck the hash computation is in mirgecom compiles, but at the same time, I can't think of a good reason to not do this. Our use case very much falls into the "many small messages" category, so this graph is pretty compelling:

(from https://cr.yp.to/siphash/siphash-20120918.pdf)

A few things need to happen in order to make this go:

• siphash24 would need to be packaged for conda-forge (presumably straightforward) (This can wait until next release and we package this for conda-forge.)
• Someone would need to go through and change the hardcoded digests.
• hexdigest should return str, not bytes dnicolodi/python-siphash24#6 (though we could hack around that)
• Specify explicit hash_constructor in call to unordered_hash loopy#849

cc @matthiasdiener

[inducer]

In addition, this will likely save considerable amounts of DRAM and disk. (Consider the "failing" test for the SQLite file that's now "too small".)

[inducer]

Python also uses siphash internally (for hash), but AFAIK that implementation is not individually accessible.