Adding __slots__ to improve performance.

We could probably significantly improve performance if we added `__slots__` to `HasTraits` and common `TraitType`. We'd just have to include `__dict__` as one of the slots to maintain backward compatibility.

Start by locating the HasTraits and common TraitType definitions in the repository and inspect how instances currently store attributes. Check the existing tests or benchmarks for these classes before assessing the compatibility impact. Done means the proposed slots preserve __dict__ behavior while demonstrating a measurable performance improvement.

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