hashtable benchmarks

[nikomatsakis]

We should add some hashtable benchmarks into our set of benchmarks. It's a rather central data structure to the compiler but also most programs. Ideally the benchmark would be targeted against a generic interface so that it can be used to compare the wealth of hashtables that we will eventually offer. =)

[thestinger]

I'm closing this since there's a generic core-map benchmark now. It would be useful to benchmark the hash separately though and compare to optimized SipHash 2-4 implementations.