Before this change, adding a lint was a difficult matter
because it always had some overhead involved. This was
because all lints would run, no matter their default level,
or if the user had #![allow]ed them. This PR changes that
fix clippy format using `cargo fmt -p clippy_{lints,utils}`
manually revert rustfmt line truncations
rename to hir::Let in clippy
Undo the shadowing of various `expr` variables after renaming `scrutinee`
reduce destructuring of hir::Let to avoid `expr` collisions
cargo fmt -p clippy_{lints,utils}
bless new clippy::author output
