Fix clippy::needless_borrow in the compiler

`x clippy compiler -Aclippy::all -Wclippy::needless_borrow --fix`.

Then I had to remove a few unnecessary parens and muts that were exposed
now.

compiler/rustc_middle/src/traits/mod.rs

@@ -686,7 +686,7 @@ impl<'tcx, N> ImplSource<'tcx, N> {
     pub fn borrow_nested_obligations(&self) -> &[N] {
         match self {
             ImplSource::UserDefined(i) => &i.nested,
-            ImplSource::Param(n) | ImplSource::Builtin(_, n) => &n,
+            ImplSource::Param(n) | ImplSource::Builtin(_, n) => n,
         }
     }