Use nightly rustfmt

[divergentdave]

We could automate and standardize a few more style choices if we used a nightly build of rustfmt, and set up a configuration file with the following options.

• format_strings = true (unstable): I made some one-off changes using this previously, to wrap very long string literals. At the time this un-stuck about half of the chains that couldn't be formatted. (the rest were due to long turbofishes)
• group_imports (unstable): Our initial convention in this repository was to group all imports together, which maps to "One". Personally I would prefer "StdExternalCrate", which creates three groups of imports separated by blank lines in that order. This is closer to what rust-analyzer's auto-import assist does, though that creates up to five groups, splitting out self and super.
• imports_granularity = "Crate" (unstable): Combines multiple import statements from the same crate into one.

[inahga]

SGTM. For local development looks like it' reasonably possible to use a stable compiler while still using nightly rustfmt rust-lang/vscode-rust#438.

[inahga]

wrap_comments may also be useful.