Unicode bidi support

[Michael_Ilseman]

@SDGGiesbrecht

However, as you suspected, bidirection is mostly a rendering issue, which is at a much higher level than the Swift language itself.

That being said, we're interested in adding the Bidi properties to Unicode.Scalar.Properties, mentioned in this post. That's a lower-level representation, but necessary for bidi processing IIUC. If you're interested in pitching that, I can help!