ci: lint test targets as well as the library and binary - #10

Summary

CI's clippy job now lints test code as well as the library and binary. It ran
cargo clippy --all-features, which never compiles test targets — which is how two
deny-level unused_io_amount errors in tests/cli/mod.rs survived from 2018 until #6's
review caught them; #8 fixed them.

Turning --all-targets on surfaced five more warnings, all in tests/integration.rs. They
are fixed here, so the job passes under the workflow's RUSTFLAGS: -Dwarnings.

The documented lint command now matches CI's. The README's was plain cargo clippy, which
has never worked on this fork: with no default backend feature it stops at the
compile_error! in src/lib.rs.

Changes

• Rename KmsSocket::{TCP, UNIX} to {Tcp, Unix}, matching the sibling KmsConnection — b765765
• Box the SecretConnection variant, 448 bytes against the other variant's 4 — cb1eef3
• Pass the message through instead of borrowing it again — c43583d
• Add --all-targets to CI's clippy job — 74f3311
• Give the same command in the README and FORK.md — 08c4ebf

Testing

• CI's command on its pinned toolchain,
  RUSTFLAGS=-Dwarnings cargo +1.89.0 clippy --all-features --all-targets: clean. On the
  base before fix(tests): write captured output in full when a command fails #8 it reported 7 errors.
• The widened job catches a warn-level lint in test code: reintroducing a single
  needless_borrow fails it.
• 64 unit + 23 integration tests pass, including the protocol tests that use the renamed and
  boxed types.