Added basic benchmark infrastructure

And also found out that 0.7.0 regresses against 0.6.0 :p

Much work to be done :D
Added benchmarks on core abstractions

Added benchmarks against the CRUD actions, via persistent.dict.
Since persistent.array acts almost identically, there's no point
in seperately benchmarking this at this stage.

The results of the benchmark can be found at
https://gist.github.com/hughfdjackson/a1f08b1fc068444a9bc1

Closes #8