Ref: https://github.com/ruby/json/issues/655
Followup: https://github.com/ruby/json/issues/657
Assuming the generator might be used for fairly small documents
we can start with a reasonable buffer size of the stack, and if
we outgrow it, we can spill on the heap.
In a way this is optimizing for micro-benchmarks, but there are
valid use case for fiarly small JSON document in actual real world
scenarios, so trashing the GC less in such case make sense.
Before:
```
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
Oj 518.700k i/100ms
JSON reuse 483.370k i/100ms
Calculating -------------------------------------
Oj 5.722M (± 1.8%) i/s (174.76 ns/i) - 29.047M in 5.077823s
JSON reuse 5.278M (± 1.5%) i/s (189.46 ns/i) - 26.585M in 5.038172s
Comparison:
Oj: 5722283.8 i/s
JSON reuse: 5278061.7 i/s - 1.08x slower
```
After:
```
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
Oj 517.837k i/100ms
JSON reuse 548.871k i/100ms
Calculating -------------------------------------
Oj 5.693M (± 1.6%) i/s (175.65 ns/i) - 28.481M in 5.004056s
JSON reuse 5.855M (± 1.2%) i/s (170.80 ns/i) - 29.639M in 5.063004s
Comparison:
Oj: 5692985.6 i/s
JSON reuse: 5854857.9 i/s - 1.03x faster
```
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