Add edsc::Indexed helper struct to act as syntactic sugar

This CL adds edsc::Indexed.

This helper class exists purely for sugaring purposes and allows writing
expressions such as:

```mlir
   Indexed A(...), B(...), C(...);
   ForNest(ivs, zeros, shapeA, ones, {
     C[ivs] = A[ivs] + B[ivs]
   });
```

PiperOrigin-RevId: 229388644