Packed tables.

Given a table like:

```
type PostInfo {
author: string,
title: string,
content: string,
create_time: string,
}
```

It's often useful to store all fields in a single key-value entry to reduce query overhead.

That's what the `@packed` attribute is for:

```
type Post {
@primary
id: string,
@packed
info: PostInfo,
}
```

However currently the `@packed` attribute is only implemented on the syntax level, and not pushed down to either the storage plan or TreeWalker.

Changes needed for `@packed`:

- Storage planner: Treat packed fields as a leaf node.
- TreeWalker: Keep track of the packed/unpacked kind of any `set` and `table` DFG nodes.
- TreeWalker: Lens-like structure for looking inside a packed field.

Start by locating the storage planner and TreeWalker entry points, then trace how set and table DFG nodes represent fields. Implement packed fields as storage-planner leaves and preserve packed/unpacked state through TreeWalker, including a lens-like way to inspect packed fields; done means all three listed behaviors are supported.

Written by the indexing model from the issue text.