Add EDSC sugar

This allows load, store and ForNest to be used with both Expr and Bindable.
This simplifies writing generic pieces of MLIR snippet.

For instance, a generic pointwise add can now be written:

```cpp
// Different Bindable ivs, one per loop in the loop nest.
auto ivs = makeBindables(shapeA.size());
Bindable zero, one;
// Same bindable, all equal to `zero`.
SmallVector<Bindable, 8> zeros(ivs.size(), zero);
// Same bindable, all equal to `one`.
SmallVector<Bindable, 8> ones(ivs.size(), one);
// clang-format off
Bindable A, B, C;
Stmt scalarA, scalarB, tmp;
Stmt block = edsc::Block({
  ForNest(ivs, zeros, shapeA, ones, {
    scalarA = load(A, ivs),
    scalarB = load(B, ivs),
    tmp = scalarA + scalarB,
    store(tmp, C, ivs)
  }),
});
// clang-format on
```

This CL also adds some extra support for pretty printing that will be used in
a future CL when we introduce standalone testing of EDSCs. At the momen twe
are lacking the basic infrastructure to write such tests.

PiperOrigin-RevId: 229375850