Try to use RString as a buffer

[byroot]

Ref: https://bugs.ruby-lang.org/issues/20878

It's definitely simpler and less code if using only high level
rb_str APIs. However the performance is really bad.

== Encoding small mixed (34 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring   270.614k i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring      2.810M (± 0.7%) i/s  (355.92 ns/i) -     14.072M in   5.008785s

Comparison:
          json-2.9.0:  6841854.5 i/s
  json-2.9.0-rstring:  2809586.8 i/s - 2.44x  slower

== Encoding small nested array (121 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring   104.936k i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring      1.066M (± 0.9%) i/s  (938.47 ns/i) -      5.352M in   5.022882s

Comparison:
          json-2.9.0:  2769443.8 i/s
  json-2.9.0-rstring:  1065559.4 i/s - 2.60x  slower

== Encoding small hash (65 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring   288.458k i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring      2.965M (± 0.7%) i/s  (337.22 ns/i) -     15.000M in   5.058436s

Comparison:
          json-2.9.0:  7606155.3 i/s
  json-2.9.0-rstring:  2965448.7 i/s - 2.56x  slower

== Encoding mixed utf8 (5003001 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring    50.000 i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring    504.537 (± 0.8%) i/s    (1.98 ms/i) -      2.550k in   5.054431s

Comparison:
          json-2.9.0:      378.8 i/s
  json-2.9.0-rstring:      504.5 i/s - 1.33x  faster

== Encoding mostly utf8 (5001001 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring    38.000 i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring    378.373 (± 0.8%) i/s    (2.64 ms/i) -      1.900k in   5.021755s

Comparison:
          json-2.9.0:      303.5 i/s
  json-2.9.0-rstring:      378.4 i/s - 1.25x  faster

== Encoding integers (8009 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring     5.373k i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring     53.530k (± 1.6%) i/s   (18.68 μs/i) -    268.650k in   5.020019s

Comparison:
          json-2.9.0:    96002.0 i/s
  json-2.9.0-rstring:    53530.1 i/s - 1.79x  slower

== Encoding activitypub.json (52595 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring     1.060k i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring     10.621k (± 0.3%) i/s   (94.15 μs/i) -     54.060k in   5.089908s

Comparison:
          json-2.9.0:    22854.8 i/s
  json-2.9.0-rstring:    10621.1 i/s - 2.15x  slower

== Encoding citm_catalog.json (500298 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring    62.000 i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring    621.649 (± 0.5%) i/s    (1.61 ms/i) -      3.162k in   5.086598s

Comparison:
          json-2.9.0:     1354.1 i/s
  json-2.9.0-rstring:      621.6 i/s - 2.18x  slower

== Encoding twitter.json (466906 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring    97.000 i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring    983.052 (± 1.0%) i/s    (1.02 ms/i) -      4.947k in   5.032796s

Comparison:
          json-2.9.0:     2175.2 i/s
  json-2.9.0-rstring:      983.1 i/s - 2.21x  slower

== Encoding canada.json (2090234 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring     1.000 i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring     19.360 (± 0.0%) i/s   (51.65 ms/i) -     97.000 in   5.010646s

Comparison:
          json-2.9.0:       20.3 i/s
  json-2.9.0-rstring:       19.4 i/s - 1.05x  slower

== Encoding many #to_json calls (2701 bytes)
ruby 3.4.0dev (2024-12-12T09:30:43Z master 197a3efc75) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
  json-2.9.0-rstring     2.108k i/100ms
Calculating -------------------------------------
  json-2.9.0-rstring     21.399k (± 1.0%) i/s   (46.73 μs/i) -    107.508k in   5.024491s

Comparison:
          json-2.9.0:    31034.7 i/s
  json-2.9.0-rstring:    21399.2 i/s - 1.45x  slower