get_ptr(mapOfMaps, key, key...)

get_ptr(mapOfMaps, key, key...)

Summary:
Since this is a not-uncommon pattern now:

```lang=cpp
if (auto found1 = get_ptr(m, k1)) {
  if (auto found2 = get_ptr(*found, k2)) {
    return use(*found2);
  }
}
```

This diff enables:

```lang=cpp
if (auto found = get_ptr(m, k1, k2)) {
  return use(*found);
}

```

Note that it works for const and non-const maps, returning a correspondingly mutable pointer.

Reviewed By: luciang, yfeldblum

Differential Revision: D3812701

fbshipit-source-id: 77ace9f5dca6cdc4cae7e6dfb9e5fc1075b5b571