rustdoc: Fix where clauses on re-exports

Projection predicates on re-exports, for the time being, are rendered as
equality predicates because that's easier. It would be nice to fix this
in the future.

Some gymnastics were needed to remove redundant bounds from the `types`
and `lifetimes` fields, remove implicit `Sized` bounds, and re-create
`?Sized` bounds.

Fix #20203, fix #20924, fix #20911, fix #20534

src/librustdoc/html/format.rs

@@ -153,8 +153,8 @@ impl<'a> fmt::String for WhereClause<'a> {
                         try!(write!(f, "{}", lifetime));
                     }
                 }
-                &clean::WherePredicate::EqPredicate => {
-                    unimplemented!()
+                &clean::WherePredicate::EqPredicate { ref lhs, ref rhs } => {
+                    try!(write!(f, "{} == {}", lhs, rhs));
                 }
             }
         }