Add PBRT-style "Normal" type ("Norm"?)

[Miles Bader]

PBRT has a distinct type for normal-vectors, which can make some code cleaner.

It's basically the same as a standard vector, but in some cases uses different operations, e.g., when transformed, it uses a "normal transform" (the transpose of the inverse of the actual transform).

Not sure the degree to which it should be interchangeable with Vec...