Add a benchmark for computing hashCodes. (#553)

Computing hash codes is incredibly slow in the current Dart protobuf
implementation. We have in-product cases where computing a hash code for
a message is nearly as slow as parsing that message from JSON. I'm
working on some improvements, so I'm adding a benchmark to track these
improvements.

Co-authored-by: Loren Van Spronsen <lorenvs@google.com>