Use tidy to enforce alphabetical dependency ordering
I get annoyed when dependencies in `Cargo.toml` files are not in alphabetical order. The [style guide](https://github.com/rust-lang/rust/blob/master/src/doc/style-guide/src/cargo.md) agrees with me.
There are ongoing efforts to provide linting/formatting of `Cargo.toml` files, e.g. https://github.com/rust-lang/rustfmt/pull/5240, https://crates.io/crates/cargo-toml-lint, and https://github.com/TimonPost/cargo-toml-format. But it's far from clear what's the right approach.
So this PR does something very simple: it uses the order checking already present in tidy. This allows incremental application of ordering, starting right now, and avoiding the need for any kind of all-at-once conversion.
If we do end up using some more comprehensive `Cargo.toml` linting/formatting solution in the future, the `tidy-alphabetical` lines will be easy to remove.
r? `@wesleywiser`
c07693c160