Bring up Appveyor for Windows CI
The current plan is to move to Appveyor for CI on Windows.
Mateusz has a first stab at a configuration. Unfortunately it looks like we will far-exceed the one-hour default build time limit. It appears they Appveyor does give extensions to open-source projects, but typically only extend to 1.5 hours. It's unlikely that this will be sufficient as even my local Windows builds take ~1.75 hours.
It appears that Rust has found a way around this as their builds routinely last ~2 hours.