Update rustfmt config to not require nightly

[FreezyLemon]

This PR updates the rustfmt configuration.. by deleting rustfmt.toml.

There are only two or three options that do anything at all. The others are set to their respective default value or otherwise ineffective.

Copied from the commit message:

Options set to their default value:
- wrap_comments
- use_small_heuristics
- fn_single_line

Unnecessary options:
- edition = "2021" (read from Cargo.toml)
- comment_width (wrap_comments = false)

Actual config changes:
- blank_lines_upper_bound: 2 -> 1
- reorder_impl_items: true -> false

This config includes unstable options which make a nightly toolchain necessary to run cargo fmt, which feels unnecessary. As you can see, there is effectively no change to the actual code formatting after this configuration change.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.91%. Comparing base (f7f2960) to head (9aebb9a). Report is 38 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #272 +/- ## ======================================= Coverage 93.91% 93.91% ======================================= Files 53 53 Lines 12269 12269 ======================================= Hits 11523 11523 Misses 746 746 ```

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[YeungOnion]

Thanks!