Array tables

Toml has array sections:

```
[[a]]
x = 1
[[a]]
x = 2
```

which parses to the same thing as:

```
a = [{"x": 1}, {"x": 2}]
```

(confirm)

I don't want to tokenize runs of two brackets separately because they occur with their normal meaning elsewhere, e.g., nested array literals: `x = [[1,2,3], [4,5,6]]` so they will have to be parsed.
