add unmanaged swift benchmark based on naive algorithm

[jgh-]

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.

[frol]

@jgh- Thank you for your contribution! I am sorry it took so long, I didn't have the bandwidth to manage it.

The performance on the laptop that all the previous benchmarks were done is the following:

• naive - 2s
• unmanaged - 0.75s

NOTE: It seems that Swift 5.0 produced the binary that is slightly slower than Swift 4.1