Consider using `package.metadata` to configure allow/warn/deny/forbid lint settings

[Predrag Gruevski]

While we wait for external tools support for the lints table, consider using package.metadata to allow crates to configure their lint settings.

For example, this could address the tokio use case where they'd prefer to allow minor lints like #[must_use]. It would also allow us to add allow-by-default lints like "API got extended," which may be desirable for some use cases and not desirable in others.

Docs: https://doc.rust-lang.org/cargo/reference/manifest.html#the-metadata-table

[Pavan Kumar Sunkara]

Related issue: #58