Prevent extraneous default construction when decoding maps

The `operator[]` on `std::map` will default construct the value if it
does not exist, and then move assign it the requested value.

By using the C++17 method `insert_or_assign`, we prevent that extra
object construction.

Author: brodeuralexis

include/fine.hpp

@@ -667,7 +667,7 @@ struct Decoder<std::map<K, V, Compare, Alloc>> {
       }
     }();
 
-    ERL_NIF_TERM key, value;
+    ERL_NIF_TERM key_term, value_term;
     ErlNifMapIterator iter;
     if (!enif_map_iterator_create(env, term, &iter,
                                   ERL_NIF_MAP_ITERATOR_FIRST)) {
@@ -677,8 +677,12 @@ struct Decoder<std::map<K, V, Compare, Alloc>> {
     // Define RAII cleanup for the iterator
     auto cleanup = IterCleanup{env, iter};
 
-    while (enif_map_iterator_get_pair(env, &iter, &key, &value)) {
-      map[fine::decode<K>(env, key)] = fine::decode<V>(env, value);
+    while (enif_map_iterator_get_pair(env, &iter, &key_term, &value_term)) {
+      auto key = fine::decode<K>(env, key_term);
+      auto value = fine::decode<V>(env, value_term);
+
+      map.insert_or_assign(std::move(key), std::move(value));
+
       enif_map_iterator_next(env, &iter);
     }
 
@@ -899,7 +903,7 @@ template <typename... Args> struct Encoder<std::tuple<Args...>> {
 template <typename T, typename Alloc> struct Encoder<std::vector<T, Alloc>> {
   static ERL_NIF_TERM encode(ErlNifEnv *env,
                              const std::vector<T, Alloc> &vector) {
-    auto terms = std::vector<ERL_NIF_TERM, fine::Allocator<ERL_NIF_TERM>>();
+    auto terms = std_vector<ERL_NIF_TERM>();
     terms.reserve(vector.size());
 
     for (const auto &item : vector) {
@@ -915,8 +919,8 @@ template <typename K, typename V, typename Compare, typename Alloc>
 struct Encoder<std::map<K, V, Compare, Alloc>> {
   static ERL_NIF_TERM encode(ErlNifEnv *env,
                              const std::map<K, V, Compare, Alloc> &map) {
-    auto keys = std::vector<ERL_NIF_TERM, fine::Allocator<ERL_NIF_TERM>>();
-    auto values = std::vector<ERL_NIF_TERM, fine::Allocator<ERL_NIF_TERM>>();
+    auto keys = std_vector<ERL_NIF_TERM>();
+    auto values = std_vector<ERL_NIF_TERM>();
 
     for (const auto &[key, value] : map) {
       keys.push_back(fine::encode(env, key));