Add rustfmt.toml to describe format

[TheButlah]

I wanted to modify it from the defaults to be:

max_width = 88  # follow python black and improve readability for side-by-side editing
hard_tabs = true # allow users to reconfigure indent width

but I didn't want to be that guy 😅 so I left it as what you already had.

[carl-anders]

I don't see the point of having an empty file and rather keep the default settings. Thanks anyways :)

[TheButlah]

Even just an empty file is helpful because it enforces the format and signals what you want, otherwise people's autoformatters will use their default format

https://www.reddit.com/r/rust/comments/9jl6a9/pro_tip_if_you_use_cargo_fmtrustfmt_use_a/