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ARROW-4681: [Rust] [DataFusion] Partition aware data sources

This PR changes the data source API to support data sources that have partitions. The `scan` method now returns a `Vec<>` of iterators.

For now the query execution only works for single partitions.

Author: Andy Grove <andygrove73@gmail.com>

Closes #3849 from andygrove/ARROW-4681 and squashes the following commits:

8b6d591 <Andy Grove> Execution engine throws error if partition count is not one
758d767 <Andy Grove> First pass at partition aware data sources