Guard the rustfmt config that rustfmt reads, and default to field init shorthand

[jph00]

cargo fastfmt now puts its disable_all_formatting guard in the rustfmt.toml that rustfmt reads for each package: the nearest one from the package directory up to the repository root. It creates a new file in the package directory only when there is none. Previously it created a guard-only rustfmt.toml beside every sub-package's Cargo.toml, even when the root guard already covered it, and those files hid root settings such as max_width.

fastfmt also runs rustfmt with use_field_init_shorthand = true, which writes Foo { x: x } as Foo { x }. Like use_small_heuristics, it overrides the project's rustfmt.toml. Projects no longer need to set it themselves.

See "Install and use" in the README.