Lazily compute whether attribute changed in _attributeModified(_:old:new:)

[jmschonfeld]

_attributeModified is called when an attribute value in an attribute storage dictionary was replaced by a new value. It is responsible for enforcing the invalidation constraints on attributes by removing any dependent attributes from the storage (recursively). Currently (assuming both a new and old value exist) this function unconditionally compares the new and old values to return early if the value didn't change (meaning attributes shouldn't be invalidated). However, this requires unboxing the existential attribute values and calling through to their == implementation which can be expensive. Instead, we can make this calculation lazy, only performing it when we actually find another attribute dependent upon the changed one. In cases where there are no dependent attributes, we avoid the equality check altogether as it is unnecessary. This shows about a 6% to 11% improvement in the setAttribute benchmark:

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│          Throughput (# / s) (K)          │        p0 │       p25 │       p50 │       p75 │       p90 │       p99 │      p100 │   Samples │
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│                   main                   │       141 │       140 │       139 │       137 │       135 │       125 │       122 │       138 │
├──────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│                  branch                  │       150 │       148 │       147 │       146 │       145 │       142 │       136 │       147 │
├──────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│                    Δ                     │         9 │         8 │         8 │         9 │        10 │        17 │        14 │         9 │
├──────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│              Improvement %               │         6 │         6 │         6 │         7 │         7 │        14 │        11 │         9 │
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[jmschonfeld]

@swift-ci please test