Added constant index operator to VectorMap.

source/mijin/container/vector_map.hpp

@@ -135,6 +135,11 @@ public:
         return emplace(key, TValue()).first->second;
     }
     
+    const TValue& operator[](const TKey& key) const
+    {
+        return at(key);
+    }
+    
     [[nodiscard]]
     iterator begin() MIJIN_NOEXCEPT { return {keys_.data(), values_.data()}; }
     [[nodiscard]]
@@ -176,6 +181,11 @@ public:
     size_type size() const MIJIN_NOEXCEPT { return keys_.size(); }
     [[nodiscard]]
     size_type max_size() const MIJIN_NOEXCEPT { return std::min(keys_.max_size(), values_.max_size()); }
+    void reserve(std::size_t size)
+    {
+        keys_.reserve(size);
+        values_.reserve(size);
+    }
     
     void clear()
     {
@@ -241,7 +251,7 @@ public:
         {
             if (keys_[idx] == key)
             {
-                return iterator(&keys_[idx], &values_[idx]);
+                return const_iterator(&keys_[idx], &values_[idx]);
             }
         }
         return end();