SIMD Interface should not provide `Index`

The concept of an Index type isn't really feasible. E.g. it attempts to be an integer simd type usable for indexing of the same width as the original simd type, that also fits into into a simd register. This isn't quite possible e.g. if the original type has char elements.

The datapar stuff the got into Parallism TS recently does not have such an index type. Instead it allows to construct simd types of a given element type and width. These types may no longer fit into a single register, but I guess that is what is necessary for correctness.

I believe the only uses for the Index type currently is in the luDecomposition() of DenseMatrix, but possibly also in ISTL.

I'd like to get this out and possibly replace it before it becomes part of an official release.