Duplicate keys in map<K, V> leak handles #14812

Right now the semantics of map<K, V> mean that if duplicate keys are supplied then the last-specified key is the one present in the map. This is generally fine but this means that Wasmtime does not provide embedders the means to correctly operate with maps-of-handles because the embedder can't ever properly deallocate the handles it was given. For example if the host is given two own handles it'll only actually receive one of them, meaning the other effectively leaks.