`Attribute` is mostly immutable, but the deserialized value is
memoized, and that value can potentially be mutable, and mutated.

When the value is immutable however, we can share instances.

Benchmark: https://gist.github.com/casperisfine/ae56bec1e7eecbff3a696b367e2bafa2

Before:

```
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
        ActiveRecord     4.664k i/100ms
Calculating -------------------------------------
        ActiveRecord     46.983k (± 1.0%) i/s   (21.28 μs/i) -    237.864k in   5.063292s
```

After:

```
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
        ActiveRecord     5.404k i/100ms
Calculating -------------------------------------
        ActiveRecord     53.502k (± 1.4%) i/s   (18.69 μs/i) -    270.200k in   5.051328s
```