[ruby/json] Reduce encoding benchmark size

Profiling revealed that we were spending lots of time growing the buffer.
Buffer operations is definitely something we want to optimize, but for
this specific benchmark what we're interested in is UTF-8 scanning performance.

Each iteration of the two scaning benchmark were producing 20MB of JSON,
now they only produce 5MB.

Now:

```
== Encoding mostly utf8 (5001001 bytes)
ruby 3.4.0dev (2024-10-18T19:01:45Z master ruby/json@7be9a333ca) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json    35.000 i/100ms
                  oj    36.000 i/100ms
           rapidjson    10.000 i/100ms
Calculating -------------------------------------
                json    359.161 (± 1.4%) i/s    (2.78 ms/i) -      1.820k in   5.068542s
                  oj    359.699 (± 0.6%) i/s    (2.78 ms/i) -      1.800k in   5.004291s
           rapidjson     99.687 (± 2.0%) i/s   (10.03 ms/i) -    500.000 in   5.017321s

Comparison:
                json:      359.2 i/s
                  oj:      359.7 i/s - same-ish: difference falls within error
           rapidjson:       99.7 i/s - 3.60x  slower
```

ruby/json@1a338532d2

Author: byroot

benchmark/encoder.rb

@@ -59,11 +59,9 @@ def benchmark_encoding(benchmark_name, ruby_obj, check_expected: true, except: [
 benchmark_encoding "small nested array", [[1,2,3,4,5]]*10
 benchmark_encoding "small hash", { "username" => "jhawthorn", "id" => 123, "event" => "wrote json serializer" }
 
-# On this one we're a bit slower (~25%).
-benchmark_encoding "mostly utf8", ([("€" * 3333)] * 2000), except: %i(json_state)
-
-# On these three benchmarks we perform well. Either on par or very closely faster/slower
-benchmark_encoding "mixed utf8", ([("a" * 5000) + "€" + ("a" * 5000)] * 2000), except: %i(json_state)
+# On these benchmarks we perform well. Either on par or very closely faster/slower
+benchmark_encoding "mixed utf8", ([("a" * 5000) + "€" + ("a" * 5000)] * 500), except: %i(json_state)
+benchmark_encoding "mostly utf8", ([("€" * 3333)] * 500), except: %i(json_state)
 benchmark_encoding "twitter.json", JSON.load_file("#{__dir__}/data/twitter.json"), except: %i(json_state)
 benchmark_encoding "citm_catalog.json", JSON.load_file("#{__dir__}/data/citm_catalog.json"), except: %i(json_state)
 

ext/json/generator/generator.c

@@ -71,7 +71,7 @@ static void convert_UTF8_to_JSON(FBuffer *out_buffer, VALUE str, const char esca
                 }
                 case 3: {
                     unsigned char b2 = ptr[pos + 1];
-                    if (out_script_safe && b2 == 0x80) {
+                    if (RB_UNLIKELY(out_script_safe && b2 == 0x80)) {
                         unsigned char b3 = ptr[pos + 2];
                         if (b3 == 0xA8) {
                             FLUSH_POS(3);