This saves one pointer in `struct set_table`, which would allow
`Set` objects to still fit in 80B TypedData slots even if RTypedData
goes from 32B to 40B large.
The existing set benchmark seem to show this doesn't have a very
significant impact. Smaller sets are a bit faster, larger sets
a bit slower.
It seem consistent over multiple runs, but it's unclear how much
of that is just error margin.
```
compare-ruby: ruby 3.5.0dev (2025-08-12T02:14:57Z master 428937a536) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-12T07:22:26Z set-entries-bounds da30024fdc) +YJIT +PRISM [arm64-darwin24]
warming up........
| |compare-ruby|built-ruby|
|:------------------------|-----------:|---------:|
|new_0 | 15.459M| 15.823M|
| | -| 1.02x|
|new_10 | 3.484M| 3.574M|
| | -| 1.03x|
|new_100 | 546.992k| 564.679k|
| | -| 1.03x|
|new_1000 | 49.391k| 48.169k|
| | 1.03x| -|
|aref_0 | 18.643M| 19.350M|
| | -| 1.04x|
|aref_10 | 5.941M| 6.006M|
| | -| 1.01x|
|aref_100 | 822.197k| 814.219k|
| | 1.01x| -|
|aref_1000 | 83.230k| 79.411k|
| | 1.05x| -|
```
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