Add a lint for writing `#[feature]` for stable features, warn by default.

Add a lint for writing `#[feature]` for stable features, warn by default.

The 'stable_features' lint helps people progress from unstable to
stable Rust by telling them when they no longer need a `feature`
attribute because upstream Rust has declared it stable.

This compares to the existing 'unstable_features', which is used
to implement feature staging, and triggers on *any* use
of `#[feature]`.