New lint: Suggest using `Self::Variant` instead of `TheType::Variant`

[Centril]

Since Rust 1.37.0 (hits stable on August 15th 2019) it is possible to use Self:: inside implementations to refer to variants of the Self type. This was stabilized in rust-lang/rust#61682 (see it for tests).

Similar to the suggestion to match and construct via Self(...) or Self { ... } it would be likewise good to suggest matching and constructing through Self::Variant where applicable. This seems consistent with Clippy's sensibilities... :)