Separate out a hir::Impl struct

This makes it possible to pass the `Impl` directly to functions, instead
of having to pass each of the many fields one at a time. It also
simplifies matches in many cases.

clippy_lints/src/methods/mod.rs

@@ -1626,7 +1626,7 @@ impl<'tcx> LateLintPass<'tcx> for Methods {
         let self_ty = cx.tcx.type_of(def_id);
 
         // if this impl block implements a trait, lint in trait definition instead
-        if let hir::ItemKind::Impl { of_trait: Some(_), .. } = item.kind {
+        if let hir::ItemKind::Impl(hir::Impl { of_trait: Some(_), .. }) = item.kind {
             return;
         }