math/vector: add norm, clean up.
math/vector: add norm, clean up.

New norm, normalized operators.

Along the way, it became clear that the operators that claimed to take
any type V were biting off more generality than they could chew.  I've
specialized them to Vector types only.

Docs expanded and clarified.

cbiffle/etl,cbiffle/etl,cbiffle/etl