Fixed axis length support similiar to `Eigen`

Start by reviewing the Dimension and ArrayRepr traits, including the possible Data refactor, and the FixedIx2 design described in the issue. Check the constraints around const_evaluatable_checked, specialization, and mixing fixed and dynamic axes; done would require an agreed design and implementation plan for fixed axis lengths.

In Eigen, We can write Matrix<float, 4, 4> to represent a matrix with fixed 4x4 size. In Rust, I would imaging syntax similar to ArrayBase<Array2Repr<T, 4, 4>, FixedIx2<4, 4>>, which has no allocation, const bound check and enable better simd instructions.

I can see several difficulty implementing this:

1. Dimension trait currently requires Add/Sub/Mul, which doesn't make sense in fixed length.
2. ArrayRepr requires exact generic argument match to FixedIx2. This might requires refactor to Data or Dimension traits.
3. Enable better simd instructions on specific axis length might require #[feature(const_evaluatable_checked)] and #[feature(specialization)], and both of them are incomplete feature.
4. Mixing fixed axis length and dynamic axis length in one matrix seems hard.