Add benchmarks

[andrewthad]

In the situations that this library was designed for, this should outperform the Data.ByteString.Builder module that comes with bytestring. These situations include things where you must repeatedly execute the same builder over and over to generate lot of small (between 32 and 1000 bytes) byte sequences concatenates. This includes most CSVs, carbon's line protocol, and influxdb's line protocol. I need to write a benchmark to see if my suspicions are actually correct.