Issue 6989 - Create a better string representation of Tid's for easier thread identification and debugging.

http://d.puremagic.com/issues/show_bug.cgi?id=6989

I'm thinking of actually implementing an `opEquals` method as well, which simply would compare the `threadID` fields.

Edit: Nevermind about `opEquals`, it's unnecessary because this is automatic thanks to the `.tupleof == .tupleof` rewrite on `==` aggregate expressions.

Maybe there's an even better way to implement toString, perhaps by inspecting `mbox` or some other field. But I figured this would be the simplest way to do it without depending on OS-specific thread identifiers.