Isolate Windows codebase from UN*X systems about swift-system

[compnerd]

I have a series of patches that I hope to finish uploading soon. That at least makes the test suite build; there are a few failures and one of them is particularly worrisome (assertion failure in FilePath).

[Jumhyn]

The test suite in this package is currently completely broken on Windows.

Is this why Windows is not currently part of the PR checks for this repo? That seems to me like the most obvious near-term way to avoid breaking the Windows port. Also, until Windows is part of the regular PR checks, it seems to me like there should be some notes in the documentation about what the status of this project is with respect to Windows.

That might take the form of:

• Elaborating in the README on how close swift-system is to the "vision" described of being the home for system interfaces "for all supported Swift platforms"—is Windows currently considered a supported platform for swift-system?
• Explicitly noting in the README that there is a lower stability guarantee for Windows than there is for other platforms, and encourage users on Windows to pin to a specific release
• Describing in CONTRIBUTING.md exactly what the expectations are with respect to testing that the Windows port

[lorentey]

Scheduling for 1.4.0; I think it's time to do this.