Update rustfmt configuration for current toolchains

[ecton]

The goal of this PR is to configure rustfmt so that it can safely be run on this project, and bundle any associated formatting changes into one PR.

[ecton]

The output of the stable version of rustfmt after removing max_width was:

Warning: Unknown configuration option `fn_args_density`
Warning: Unknown configuration option `reorder_extern_crates`
Warning: Unknown configuration option `reorder_extern_crates_in_group`
Warning: Unknown configuration option `reorder_imported_names`
Warning: Unknown configuration option `reorder_imports_in_group`
Warning: can't set `spaces_around_ranges = true`, unstable features are only available in nightly channel.
Warning: can't set `error_on_line_overflow = false`, unstable features are only available in nightly channel.

error_on_line_overflow doesn't appear to affect this project, but spaces_around_ranges requires the nightly version of rustfmt. I removed the options that are unknown. Do you want to keep the requirement on using the nightly toolchain?

The only reason I ask is because I'm using rust-analyzer in my daily development, and I can't seem to configure it to work with the nightly toolchain (short of building it myself). Still, I'm willing to run cargo +nightly fmt before pushing changes if you would prefer not to change that.

Let me know what your thoughts are!

[kvark]

Thank you for the PR!
We should definitely be using the nightly rustfmt.