Tracking `rustfmt` unstable options

[mattheworris]

This is a tracking issue for rustfmt features that should be stabilized.
Frequency currently uses these features and therefore requires a nightly version.
When these options are stabilized, we can remove our dependence on nightly.

• [unstable option] imports_granularity rust-lang/rustfmt#4991
  • grouping of use statements: current option Crate
• [unstable option] reorder_impl_items rust-lang/rustfmt#3363
  • type, const first, then macros and methods: current option: false,
• [unstable option] spaces_around_ranges rust-lang/rustfmt#3367
  • spaces around the .., ..=, and ... range operators: current option false
• [unstable option] binop_separator rust-lang/rustfmt#3368
  • where to put binary operator on mulitline: current option Back
• [unstable option] match_arm_blocks rust-lang/rustfmt#3373
  • whether arm bodies are wrapped: current option: false
• [unstable option] trailing_semicolon rust-lang/rustfmt#3378
  • semicolon after break, continue and reture: current option false
• [unstable option] trailing_comma rust-lang/rustfmt#3379
  • trailing comma for lists: current option Vertical