lint traits with only one implementor

Traits are awesome, as they allow us to decouple the behavior from the data.

However, sometimes they may be overused, leading to indirection and making it harder to read the code than strictly necessary (because one has to look up the trait impls; and there is always the risk other code may also implement those traits).

So, I'd like to (allow for now) lint traits that have only one implementing type, to allow people to quickly identify extension points that have not yet been used, so they can get rid of them and make life easier for rustc and fellow coders.

The lint could either be split in two (for public/private traits), where the former would be allowed, or we allow for aboolean config flag.

The issue names no files, tests, or entry points, so first review the rust-clippy repository structure and existing lint implementations. Clarify whether the work should cover public and private traits and how the proposed configuration should behave; done means an agreed design, implementation, and tests for traits with exactly one implementing type.