add unmanaged swift benchmark based on naive algorithm#76

This also adds a Package.swift so that you can build multiple benchmarks at once using `swift build` and updates the random function so that it can be built on both Linux and macOS. I see about a 3.5x speedup on macOS using unmanaged vs managed. It's possible this could be further tuned - using loops instead of recursion and other tricks to squeeze out more performance but I think just turning off arc results in a much larger performance increase than other strategies would.