Clean up imports and crate attributes

[Inityx]

The imports in most files appear to have been incompletely converted from edition 2015 to edition 2018, with some duplicates and repeated prefixes. Additionally, some crate attributes were set separately when they could have been set jointly.

This change cleans up imports and consolidates crate attributes. Some import sections have been annotated #[rustfmt::skip] and manually formatted for readability.

[Inityx]

It appears that the rustfmt imports_layout config is only available on nightly. The default formatting mode produces undesirable output, so that stabilization should be considered a blocker for this PR unless the repository owners are willing to #[rustfmt::skip] imports.

[dtwood]

Thanks for the MR, all LGTM although it's a shame that we can't merge because of an unstable rustfmt feature.

Looking through the diffs from the rustfmt check, I don't actually find the version without imports_layout too bad, so would be happy for you to either push again after removing the config and re-running rustfmt, or to add #[rustfmt::skip] to the places that need it.

Two other things that I think should be possible if you want to add them:

• extern crate panic_halt -> use panic_halt as _
• pub use crate::gpio::GpioExt as _tm4c123x_hal_gpio_GpioExt -> pub use crate::gpio::GpioExt as _

[Inityx]

I have applied your recommendations, and am happy with the formatting I landed on. 😃
Also TIL that use x as _; can replace extern crate x; in link-only scenarios!

[dtwood]

Thanks again for the MR, merged :-)