Weaken records in let-bindings

This change also means we must update the interface of `Dict.empty` and
`Set.empty` from

```
Dict.empty : Dict k v
```

to

```
Dict.empty : {} -> Dict k v
```

crates/compiler/constrain/src/expr.rs

@@ -3833,13 +3833,13 @@ fn is_generalizable_expr(mut expr: &Expr) -> bool {
             | RunLowLevel { .. }
             | ForeignCall { .. }
             | EmptyRecord
+            | Expr::Record { .. }
             => {
                 return false
             }
             // TODO(weakening)
             Var(_, _)
             | AbilityMember(_, _, _)
-            | Expr::Record { .. }
             | Crash { .. }
             | Access { .. }
             | Accessor(_)