Write scoring code for microbenchmarks

As part of the work for implementing the microbenchmark suite, we should design and implement classes for both individual iteration scores (including the wall-clock runtime of the task and the number of MB processed) and individual benchmark scores (including a sorted array of iteration scores, a set of benchmark types corresponding to the given benchmark, and a method for getting the _n_th percentile of the benchmark). Additionally, we'll need to implement functionality for doing composite score calculations.