[red-knot] optimize building large unions of literals (#17403)
## Summary
Special-case literal types in `UnionBuilder` to speed up building large
unions of literals.
This optimization is extremely effective at speeding up building even a
very large union (it improves the large-unions benchmark by 41x!). The
problem we can run into is that it is easy to then run into another
operation on the very large union (for instance, narrowing may add it to
an intersection, which then distributes it over the intersection) which
is still slow.
I think it is possible to avoid this by extending this optimized
"grouped" representation throughout not just `UnionBuilder`, but all of
our union and intersection representations. I have some work in this
direction, but rather than spending more time on it right now, I'd
rather just land this much, along with a limit on the size of these
unions (to avoid building really big unions quickly and then hitting
issues where they are used.)
## Test Plan
Existing tests and benchmarks.
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
