Pure formatting pass from `cargo fmt --all`. No logic changes. Separating
this out so the 1.9 release feature commits that follow show only their
intentional edits.
Add rustlang feature flag to wfe-yaml with support for all cargo and
rustup step types (15 total), including cargo-doc-mdx.
Schema additions: output_dir, package, features, all_features,
no_default_features, release, profile, toolchain, extra_args,
components, targets, default_toolchain fields on StepConfig.
Integration tests for compiling all step types from YAML, and
containerd-based end-to-end tests for running Rust toolchain
inside containers from bare Debian images.
