Adding micro benchmark suite

This is an initial go at putting together a small micro-benchmarks suite to compare the performance of the backends and the performance of changes to them (eg. track improvements/regressions). The benchmark suite is configured and built almost identically to how the tests are setup, for example the vtkm_save_benchmarks and vtkm_benchmarks are essentially just vtkm_save_worklets and vtkm_worklet_unit_tests. However the benchmarks aren't actually added to the list of tests so they won't run with make test and can be run seperately if desired. They'll also only be built if VTKm_ENABLE_BENCHMARKS and VTKm_ENABLE_TESTING are on.

I'm curious what folks think about these benchmarks, do they measure useful things? Does the test data look ok? Is it worth it to run all the various data sets for sort? Any other comments are great as well of course.

Thank you!