Configure `cargo fmt` for consistent formatting of imports

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
Our imports are very inconsistent and are not currently formatted by `cargo fmt` other than reordering within groups.

**Describe the solution you'd like**

I would like to add a `rustfmt.toml` with these settings:

``` toml
merge_imports = true
group_imports = "StdExternalCrate"
```

Here is an example of imports formatted with these settings, using the command `cargo +nightly fmt`:

```rust
use std::{
collections::{HashMap, HashSet},
convert::TryFrom,
fmt::{Display, Formatter},
sync::Arc,
};

use arrow::{
compute::can_cast_types,
datatypes::{DataType, Field, Schema, SchemaRef},
};

use crate::{
error::{DataFusionError, Result, SchemaError},
field_not_found, Column,
};
```

To minimize disruption, we could do this one crate or module at a time.

**Describe alternatives you've considered**
None

**Additional context**
None

Start by reviewing the repository's crate and module layout, then check how `cargo +nightly fmt` formats the existing imports. Add a `rustfmt.toml` with the proposed settings and apply it one crate or module at a time; done means imports are consistently grouped and formatted without unnecessary disruption.

Written by the indexing model from the issue text.