Use Python data structures to define datashape

We currently use lots of internal data structures like `Tuple`, `Record` and `DataShape` to construct datashapes. Maybe we can get away with `tuple`, `ordereddict`, and `list`?

```
`10 * var * {name: string}`

DataShape([10, var, OrderedDict([['name', 'string']]))
```

This might make traversing these data structures much cleaner. I think that this choice should be mostly invisible to the user.