AK: Make it possible to move and copy HashMap and HashTable.

Previously it was only possible to move them, but we should allow copying
as well, since it's gonna be useful for many things.

AK/HashTable.h

@@ -22,7 +22,23 @@ private:
 
 public:
     HashTable() {}
-    explicit HashTable(HashTable&& other)
+    HashTable(const HashTable& other)
+    {
+        ensure_capacity(other.size());
+        for (auto& it : other)
+            set(it);
+    }
+    HashTable& operator=(const HashTable& other)
+    {
+        if (this != &other) {
+            clear();
+            ensure_capacity(other.size());
+            for (auto& it : other)
+                set(it);
+        }
+        return *this;
+    }
+    HashTable(HashTable&& other)
         : m_buckets(other.m_buckets)
         , m_size(other.m_size)
         , m_capacity(other.m_capacity)