[Windows] Build and test on Windows.

[jeffdav]

Issue #, if available: #289

Description of changes:

• Add posix and bsd paths to excludes for Windows.
• Define AWS_OS_WINDOWS_DESKTOP and AWS_USE_IO_COMPLETION_PORTS for Windows.
• Fix some type casts between Int32 <--> UInt32 that cause errors on Windows.
• Remove non-existent exclude path (silences a useless warning).
• Explicitly use Foundation.UUID where needed to avoid confusion with Windows SDK UUID.
• Add setenv/unsetenv shim for tests on Windows.

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[jeffdav]

Still has two build issues:

1. When building AwsCommonRuntimeKit, the aws-c-cal header opensslcrypto_common.h is picked up by the clang importer, even though it's excluded in Package.swift. I fixed this locally by putting #if _MSC_VER around the whole header. I think the proper fix is to provide a module.modulemap file in aws-c-cal.
2. When building AwsCommonRuntimeKit, the aws-c-common module fails to find GetConsoleMode(), despite including windows.h, which the documentation says should be enough. I fixed it locally by including consoleapi.h as well.

Since these issues are in submodules, they may need to be fixed first, but I'd love feedback.

Also some tests fail, but I think that's because I just don't have the local environment setup correctly.

[jeffdav]

I'm going to close this for now, since it's just a POC.