introduces Mirror.typeOf

I just realized that our tag-based shortcuts were incomplete, because
they only work with root mirrors (doing just u.typeTag[T].tpe means that
the type is going to be resolved in u.rootMirror because that's the default).

This commit fixes this oversight. I'm hoping for 2.11.0-RC3, but also feel
free to reschedule to 2.12.0-M1 if it becomes clear that RC3 isn't happening.