(PUP-2758) Generate tasks with arguments defined by benchmark

This makes it possible for a benchmark scenario that wants
additional details to run a smaller portion of the benchmark to
configure these by defining a class called BenchmarkerTask (currently
only used by the evaluations benchmark). The separation from the
Benchmarker class is that this class is loaded when rake generates a
list of tasks, and it is not wanted to load each full benchmark in turn
to generate this list. (Over time, the BenchmarkerTask can be expanded
on to perform other task generation duties).

It is now possible to run all of the evaluations benchmarks,
profile all, run one particular micro benchmark, or just its parse or
evaluation part by suffixing the name with _parse or _eval. As an
example rake benchmark:evaluations:run[assert_type_eval] runs only the
evaluation part of the assert_type micro benchmark in evaluations. (This
is especially useful when running profiling.

For profiling, the parameter warm_up_runs is always included.