Change arithmetic benchmark to output throughput values + Improve coding style in benchmarks

@jpihl @mortenvp I changed the arithmetic benchmarks to output throughput values instead of times. I also changed the names of the operations to "friendly" strings (those are a lot easier to use). 
We should be able to easily compare the master against the new mixin-layer implementation by looking at the throughputs. We can also merge this into Jeppe's branch, then the coding style improvements are complete for the entire fifi codebase. Hopefully, the merge conflicts will be minimal.