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/solve/tests/solve_expr.rs

@@ -8476,10 +8476,10 @@ mod solve_expr {
 
                 main =
                     s1 : Set U8
-                    s1 = Set.empty
+                    s1 = Set.empty {}
 
                     s2 : Set Str
-                    s2 = Set.empty
+                    s2 = Set.empty {}
 
                     Bool.isEq s1 s1 && Bool.isEq s2 s2
                 #   ^^^^^^^^^          ^^^^^^^^^