Pass in the type as a string

### Feature Request / Improvement

Right now we expect to pass in the actual string:

```python
schema = Schema(
    NestedField(1, 'foo', StringType())
)
```

I think it would be nice to also allow strings:

```python
schema = Schema(
    NestedField(1, 'foo', 'string')
)
```

At the same time, we should reject complex types (`list/map/struct`).