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ARROW-5946: [Rust] [DataFusion] Fix bug in projection push down logic

There was a dumb bug when deriving the projected schema. The following code was used:

```
for i in in 0..projection.len()
```

This should have been

```
for i in in 0..&projection
```

This PR modifies the `TableScan` variant of `LogicalPlan` to store both the underlying table schema, and the projected schema. I added a unit test to demonstrate that the bug is now fixed.

Author: Andy Grove <andygrove73@gmail.com>

Closes #4878 from andygrove/ARROW-5946 and squashes the following commits:

8ac74c81d <Andy Grove> remove println
a992c1703 <Andy Grove> Fix bug in projection push down