Only store key once when keys are equal

[al8n]

Example:

insert `k1 = "foo", v1 = "foo1"`, got pointer1

insert `k1 = "foo", v2 = "foo2"`, got pointer2

In the above example, for the second entry, we do not need to store k1 again, we can make the key pointer of the returned pointer2 points the same old k1 to save space.