Elide Generator::State allocation until a `to_json` method has to be called

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

For very small documents, the biggest performance gap with alternatives is
that the API impose that we allocate the `State` object. In a real world app
this doesn't make much of a difference, but when running in a micro-benchmark
this doubles the allocations, causing twice the amount of GC runs, making us
look bad.

However, unless we have to call a `to_json` method, the `State` object isn't
visible, so with some refactoring, we can elude that allocation entirely.

Instead we allocate the State internal struct on the stack, and if we need
to call a `to_json` method, we allocate the `State` and spill the struct on
the heap.

As a result, `JSON.generate` is now as fast as re-using a `State` instance,
as long as only primitives are generated.

Before:
```
== Encoding small mixed (34 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
        json (reuse)   598.654k i/100ms
                json   400.542k i/100ms
                  oj   533.353k i/100ms
Calculating -------------------------------------
        json (reuse)      6.371M (± 8.6%) i/s  (156.96 ns/i) -     31.729M in   5.059195s
                json      4.120M (± 6.6%) i/s  (242.72 ns/i) -     20.828M in   5.090549s
                  oj      5.622M (± 6.4%) i/s  (177.86 ns/i) -     28.268M in   5.061473s

Comparison:
        json (reuse):  6371126.6 i/s
                  oj:  5622452.0 i/s - same-ish: difference falls within error
                json:  4119991.1 i/s - 1.55x  slower

== Encoding small nested array (121 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
        json (reuse)   248.125k i/100ms
                json   215.255k i/100ms
                  oj   217.531k i/100ms
Calculating -------------------------------------
        json (reuse)      2.628M (± 6.1%) i/s  (380.55 ns/i) -     13.151M in   5.030281s
                json      2.185M (± 6.7%) i/s  (457.74 ns/i) -     10.978M in   5.057655s
                  oj      2.217M (± 6.7%) i/s  (451.10 ns/i) -     11.094M in   5.044844s

Comparison:
        json (reuse):  2627799.4 i/s
                  oj:  2216824.8 i/s - 1.19x  slower
                json:  2184669.5 i/s - 1.20x  slower

== Encoding small hash (65 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
        json (reuse)   641.334k i/100ms
                json   322.745k i/100ms
                  oj   642.450k i/100ms
Calculating -------------------------------------
        json (reuse)      7.133M (± 6.5%) i/s  (140.19 ns/i) -     35.915M in   5.068201s
                json      4.615M (± 7.0%) i/s  (216.70 ns/i) -     22.915M in   5.003718s
                  oj      6.912M (± 6.4%) i/s  (144.68 ns/i) -     34.692M in   5.047690s

Comparison:
        json (reuse):  7133123.3 i/s
                  oj:  6911977.1 i/s - same-ish: difference falls within error
                json:  4614696.6 i/s - 1.55x  slower
```

After:

```
== Encoding small mixed (34 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
        json (reuse)   572.751k i/100ms
                json   457.741k i/100ms
                  oj   512.247k i/100ms
Calculating -------------------------------------
        json (reuse)      6.324M (± 6.9%) i/s  (158.12 ns/i) -     31.501M in   5.023093s
                json      6.263M (± 6.9%) i/s  (159.66 ns/i) -     31.126M in   5.017086s
                  oj      5.569M (± 6.6%) i/s  (179.56 ns/i) -     27.661M in   5.003739s

Comparison:
        json (reuse):  6324183.5 i/s
                json:  6263204.9 i/s - same-ish: difference falls within error
                  oj:  5569049.2 i/s - same-ish: difference falls within error

== Encoding small nested array (121 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
        json (reuse)   258.505k i/100ms
                json   242.335k i/100ms
                  oj   220.678k i/100ms
Calculating -------------------------------------
        json (reuse)      2.589M (± 9.6%) i/s  (386.17 ns/i) -     12.925M in   5.071853s
                json      2.594M (± 6.6%) i/s  (385.46 ns/i) -     13.086M in   5.083035s
                  oj      2.250M (± 2.3%) i/s  (444.43 ns/i) -     11.255M in   5.004707s

Comparison:
        json (reuse):  2589499.6 i/s
                json:  2594321.0 i/s - same-ish: difference falls within error
                  oj:  2250064.0 i/s - 1.15x  slower

== Encoding small hash (65 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
        json (reuse)   656.373k i/100ms
                json   644.135k i/100ms
                  oj   650.283k i/100ms
Calculating -------------------------------------
        json (reuse)      7.202M (± 7.1%) i/s  (138.84 ns/i) -     36.101M in   5.051438s
                json      7.278M (± 1.7%) i/s  (137.40 ns/i) -     36.716M in   5.046300s
                  oj      7.036M (± 1.7%) i/s  (142.12 ns/i) -     35.766M in   5.084729s

Comparison:
        json (reuse):  7202447.9 i/s
                json:  7277883.0 i/s - same-ish: difference falls within error
                  oj:  7036115.2 i/s - same-ish: difference falls within error

```

1 files changed, 15 insertions, 0 deletions

diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
index 57c4e6ceeb..2b1d48b61b 100755
--- a/test/json/json_generator_test.rb
+++ b/test/json/json_generator_test.rb
@@ -486,6 +486,21 @@ class JSONGeneratorTest < Test::Unit::TestCase
     end
   end
 
+  def test_to_json_called_with_state_object
+    object = Object.new
+    called = false
+    argument = nil
+    object.singleton_class.define_method(:to_json) do |state|
+      called = true
+      argument = state
+      "<hello>"
+    end
+
+    assert_equal "<hello>", JSON.dump(object)
+    assert called, "#to_json wasn't called"
+    assert_instance_of JSON::State, argument
+  end
+
   if defined?(JSON::Ext::Generator) and RUBY_PLATFORM != "java"
     def test_valid_utf8_in_different_encoding
       utf8_string = "€™"