Rework of the traits for Vectors.

The goal is to make traits less fine-grained for vectors, and reduce the amount of `use`.

- Scalar{Mul, Div} are removed, replaced by Mul<N, V> and Div<N, V>,
- Ring and DivisionRing are removed. Use Num instead.
- VectorSpace, Dot, and Norm are removed, replaced by the new, higher-level traits.

Add four traits:
- Vec: common operations on vectors. Replaces VectorSpace and Dot.
- AlgebraicVec: Vec + the old Norm trait.
- VecExt: Vec + every other traits vectors implement.
- AlgebraicVecExt: AlgebraicVec + VecExt.