Enforce module import formatting

[xyzzyz]

I did not add it to rustfmt.toml, as it requires nightly toolchain. Instead, it's enforced on CI only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.68%. Comparing base (ce0d2e1) to head (690b1c1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #503   +/-   ##
=======================================
  Coverage   97.68%   97.68%           
=======================================
  Files          20       20           
  Lines        3971     3971           
=======================================
  Hits         3879     3879           
  Misses         92       92           

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[ctz]

How about following the arrangement in the rustls repo, to summarise:

• have a .rustfmt.unstable.toml in repo root, which would contain:

group_imports = "StdExternalCrate"
imports_granularity = "Module"

• optionally, have a .cargo/config.toml which contains:

[alias]
fmt-unstable = ["fmt", "--", "--config-path", ".rustfmt.unstable.toml"]

CI runs both cargo +stable fmt --check and cargo +nightly fmt-unstable --check and fails on either. Failure on the nightly one advises "Nightly formatting check failed. Please run cargo +nightly fmt-unstable"

[ctz]

Actually, I'll do a follow-up