Add .rustfmt to unify styling

The only problem I discovered is line breaking. I really hate when `rustfmt` does the following change:

```rust
pub fn long_function(&mut self, arg1: Arg1, arg2: Arg2, arg3: T) -> Result
where
T: Display,
E: Error
{
unimplemented!()
}
```

```rust
pub fn long_function(&mut self,
arg1: Arg1,
arg2: Arg2,
arg3: T)
-> Result
where
T: Display,
E: Error
{
unimplemented!()
}
```

Start by inspecting the repository's Rust source files and the formatting behavior shown in the issue. Add the requested .rustfmt configuration so long function signatures use the preferred line breaking, then run rustfmt on the project and verify the resulting formatting is consistent.

Written by the indexing model from the issue text.