The `[hints]` table in a `Cargo.toml` manifest provides optional
information that Cargo can use for building the package, and will use
even when using the package as a dependency. All hints can be safely
ignored, and Cargo only warns about unknown hints, but does not error.
This allows packages to use hints without depending on new Cargo.
Add a `mostly-unused` hint, which allows a package to hint that most
users of the package will not use most of its items. This is useful for
improving the build performance of crates with large dependencies.
Crates can override this hint using `hint-mostly-unused = false` in
their profile for a dependency.
These tests show what prior versions of Cargo will do with hints.
The subsequent addition of support for hints will modify these tests to
reflect the corresponding changes to Cargo.
