set.c: Store set_table->bins at the end of set_table->entries

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|         -|
```

test/ruby/test_set.rb

@@ -924,6 +924,18 @@ class TC_Set < Test::Unit::TestCase
     end
     end;
   end
+
+  def test_larger_sets
+    set = Set.new
+    10_000.times do |i|
+      set << i
+    end
+    set = set.dup
+
+    10_000.times do |i|
+      assert_includes set, i
+    end
+  end
 end
 
 class TC_Enumerable < Test::Unit::TestCase