[ruby/json] Allocate the FBuffer struct on the stack

Ref: https://github.com/ruby/json/issues/655

The actual buffer is still on the heap, but this saves a pair
of malloc/free.

This helps a lot on micro-benchmarks

Before:

```
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                  Oj   531.598k i/100ms
          JSON reuse   417.666k i/100ms
Calculating -------------------------------------
                  Oj      5.735M (± 1.3%) i/s  (174.35 ns/i) -     28.706M in   5.005900s
          JSON reuse      4.604M (± 1.4%) i/s  (217.18 ns/i) -     23.389M in   5.080779s

Comparison:
                  Oj:  5735475.6 i/s
          JSON reuse:  4604380.3 i/s - 1.25x  slower
```

After:

```
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/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
```

Bench:

```ruby
require 'benchmark/ips'
require 'oj'
require 'json'

json_encoder = JSON::State.new(JSON.dump_default_options)
test_data = [1, "string", { a: 1, b: 2 }, [3, 4, 5]]

Oj.default_options = Oj.default_options.merge(mode: :compat)

Benchmark.ips do |x|
  x.config(time: 5, warmup: 2)

  x.report("Oj") do
    Oj.dump(test_data)
  end

  x.report("JSON reuse") do
    json_encoder.generate(test_data)
  end

  x.compare!(order: :baseline)
end
```

https://github.com/ruby/json/commit/72110f7992

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