[pdq][mlh] Add hashing benchmark to /bin

[Dcallies]

Add a new script in the pdq/bin directory that can help test performance. CPP seems like a reasonable language to optimize performance for.

The script should take at a minimum:

1. A list of photo files to run on (you can do a whole directory by doing something like hash_benchmark my_dir/*.jpg in bash
2. An argument -n for number of runs (if n > photos, it should repeatedly hash the same photos)
3. An --dihedral argument to also compute dihedrals

It should then at least output:

1. Total time
2. time per hash