Further encapsulate dirty checking on `Attribute`

We can skip the allocation of a full `AttributeSet` by changing the
semantics of how we structure things. Instead of comparing two separate
`AttributeSet` objects, and `Attribute` is now a singly linked list of
every change that has happened to it. Since the attribute objects are
immutable, to apply the changes we simply need to copy the head of the
list.

It's worth noting that this causes one subtle change in the behavior of
AR. When a record is saved successfully, the `before_type_cast` version
of everything will be what was sent to the database. I honestly think
these semantics make more sense, as we could have just as easily had the
DB do `RETURNING *` and updated the record with those if we had things
like timestamps implemented at the DB layer.

This brings our performance closer to 4.2, but we're still not quite
there.

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