I am going to be adding a variant of this that uses
Unmanaged._withUnsafeGuaranteedRef(...). To truly make it fair, I am
adding variants that also use an access ref entrypoint.
I am going to add a version of this that tests that Unmanaged._withUGR
gives us expected performance. So we really need both versions of all of
the four current benchmarks to make this truly fair.
Specifically, I add some benchmarks for weak, unowned, unsafe (unowned), and
unmanaged. The reason for the split in between unsafe (unowned) and unmanaged is
that one is testing the raw compiler features and the other is validating stdlib
performance.
