add rustfmt.toml and use nightly formatting

[mattsse]

Add rustfmt.toml with some configs, listed here https://rust-lang.github.io/rustfmt/

use nightly formatting cargo +nightly fmt

add separate CI task for lining

1 clippy error, could either fix here or separately

[ralexstokes]

nice work!

I like breaking out the linting into a separate CI job.

I tend to prefer just the "stock" configuration for some tool to reduce the amount of overhead needed to operate a given codebase, e.g. requiring rustfmt.toml here although I'm open to these changes

I left some questions in the comments to address before merging. And about the CI failures, I'm fine to merge this in now if you want to follow up soon with another PR fixing the lints -- from what I can see we can just add some Boxes to the offending error variants and satisfy the lint

[mattsse]

made changes to the action

most of the rustfmt.toml settings concern line, comment wrapping, and removing trailing commas semicolons
some are a bit more opinionated, but imo really only

https://rust-lang.github.io/rustfmt/?version=v1.5.1&search=#binop_separator

https://rust-lang.github.io/rustfmt/?version=v1.5.1&search=#use_field_init_shorthand

and

https://rust-lang.github.io/rustfmt/?version=v1.5.1&search=#use_small_heuristics

which allows more width for calls etc before it wraps

but happy to use the defaults instead

[ralexstokes]

im fine w/ the config you added, can you follow up with a PR to fix the lint?

if so ill go ahead and merge this in the mean time

[mattsse]

fixed by boxing the large error variant

[ralexstokes]

excellent, thanks for this!