Add an assert to wit-parser's assert_valid. (#1908)

Indexmap's [`MutableKeys` trait] allows one to mutate the keys in an
`IndexMap`, however it doesn't prevent bugs where the new keys have
different hash values from the old keys. Since I recently made this
mistake, add an assert to help catch such bugs. It doesn't always
catch the bug, because whether or not the bug is observable depends
on hashmap internals, but it's better than nothing.

[`MutableKeys` trait]: https://docs.rs/indexmap/latest/indexmap/map/trait.MutableKeys.html

crates/wit-parser/src/resolve.rs

@@ -1500,6 +1500,11 @@ package {name} is defined in two different locations:\n\
             let mut worlds = HashSet::new();
             for (name, world) in pkg.worlds.iter() {
                 assert!(worlds.insert(*world));
+                assert_eq!(
+                    pkg.worlds.get_key_value(name),
+                    Some((name, world)),
+                    "`MutableKeys` impl may have been used to change a key's hash or equality"
+                );
                 let world = &self.worlds[*world];
                 assert_eq!(*name, world.name);
                 assert_eq!(world.package.unwrap(), id);