Allow configuring lints right from Cargo.toml

[nazar-pc]

I am adding files and directories to the root of the project very conservatively.

Since Cargo.toml already allows to enable/disable lints, it would be natural to be able to configure their options right there rather than needing a separate clippy.toml file.

For example, I'd really like to be able do this:

[workspace.lints.clippy]
unwrap_used = { level = "warn", allow-unwrap-in-tests = true }