Add Windows to Dalli CI

[petergoldstein]

We've regressed Windows support multiple times, which is not a good thing. It would be ideal if we could introduce a Github workflow that ran Dalli's tests on Windows, to help ensure against such regressions. Simply running a single MRI Ruby on the Windows platform would be fine.

While it's relatively easy to get a Windows VM up and running, the real challenge is the Memcached installation and corresponding changes to the Memcached mock. This issue tracks the work required to get the tests running in CI under Windows.