math/vector: make literal constructor more robust.
math/vector: make literal constructor more robust.

This applies the same TypeList trick I used on Matrix.  It only affects
oddball vectors (5-vecs and larger), since the smaller ones already have
type-specific constructors.

Error messaging and type coercion are both way more pleasant for large
vecs now.

cbiffle/etl,cbiffle/etl,cbiffle/etl