Fix Windows build break in Terminal.

[jeffdav]

There are three things that cause Terminal to not build on Windows:

1. isprint() returns an Int32 which requires an explicit comparison. Easy fix: just compare against 0.
2. _getch() returns an Int32, but Unicode.Scalar() takes a UInt32. So we must cast.
3. The entire implementation of size was Posix-specific. Add a Windows implementation.

[jeffdav]

Note: To build on Windows, first one must update swift-nio to this revision or later:
apple/swift-nio@8b66b22

[0xTim]

@jeffdav Could you enable Windows CI so we can track this properly? See https://github.com/vapor/jwt-kit/blob/main/.github/workflows/test.yml#L35

[jeffdav]

Sure. But we may have to wait for swift-nio to ship a new minor revision.

[0xTim]

Yeah I'm not expecting it to pass but we can at least track the errors across the modules and make sure we don't add regressions in new code. It will be a while until NIO ships a windows compatible version