Avoid new lints in 1.81.0

[Techcable]

Avoid new lints:

• Avoid check-cfg lint in 1.81.0
• Rename ./cargo/config to .cargo/config.toml
• test_edition2018: Explicitly specify edition = "2018"
• Remove unused extern crate declarations, avoiding #[deny(unused_extern_crates)] lint.
  • Switch #[deny(rust_2018_idioms)] to #[warn(...)] to avoid unnecessary build failures in the future.
• Unconditionally import from alloc/core
  • Declare #[no_std] if and only if #[cfg(not(feature = "std)], avoiding an extern crate std declaration. This expresses the intent clearer.
• doc: Fix #[warn(clippy::doc_lazy_continuation)]
• ci: Update the "recent hardcoded version" to 1.81 (used for clippy lints)

Make sure to:

• [x] Add an entry to CHANGELOG.md (if necessary)

[Techcable]

Struggling with the auguwu/clippy-action action.

A couple issues:

• The --features flag isn't working when used with check-args
• We get lots of duplicate warnings by running multiple feature combinations

IDEA: Maybe run just once with --all-features?

[Techcable]

Split the clippy CI changes into PR #339