Einstein summation library

[Zenw]

https://code.dlang.org/packages/gged/~main

I have been working on extending the functionality of 
multidimensional arrays using mir.ndslice, and now einstein 
summation is available.

It may not be practical, but it can be described nicely, like 
below, so I will introduce this.

```D
     auto A  = iota(9.).array.gged!double(3,3);
     auto x  = iota(3.).gged!double(3);
     auto Ax = Einsum | A.ij*x.j; // [5, 14, 23]
```