report allocations in benchmarks, clone a gc-friendly version of mmh3, r...

...educe allocs in BenchmarkAddSparse and BenchmarkAddNormal

old allocs

BenchmarkAddSparse       1000000              3002 ns/op             449 B/op         11 allocs/op
BenchmarkAddNormal       2000000               981 ns/op             371 B/op         10 allocs/op

new allocs

BenchmarkAddSparse       1000000              2551 ns/op             213 B/op          5 allocs/op
BenchmarkAddNormal       5000000               443 ns/op             136 B/op          4 allocs/op