Use rustfmt defaults (#174)

Without this file, rustfmt's configuration might be coming from a
developer's global configuration for rustfmt. To ensure consistency, we
define an empty rustfmt configuration. This is explained upstream:

> Rustfmt is designed to be very configurable. You can create a TOML
file called `rustfmt.toml` or `.rustfmt.toml`, place it in the project
or any other parent directory and it will apply the options in that
file. If none of these directories contain such a file, both your home
directory and a directory called `rustfmt` in your [global config
directory](https://docs.rs/dirs/1.0.4/dirs/fn.config_dir.html) (e.g.
`.config/rustfmt/`) are checked as well.

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