Use a table to check for escaping needs

This performs noticeably better than the boolean logic.

Before:

```
== Encoding twitter.json (466906 bytes)
ruby 3.4.0preview2 (2024-10-07 master 32c733f57b) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json   189.000 i/100ms
                  oj   228.000 i/100ms
           rapidjson   108.000 i/100ms
Calculating -------------------------------------
                json      1.903k (± 1.2%) i/s  (525.55 μs/i) -      9.639k in   5.066521s
                  oj      2.306k (± 1.3%) i/s  (433.71 μs/i) -     11.628k in   5.044096s
           rapidjson      1.069k (± 2.4%) i/s  (935.38 μs/i) -      5.400k in   5.053794s

Comparison:
                json:     1902.8 i/s
                  oj:     2305.7 i/s - 1.21x  faster
           rapidjson:     1069.1 i/s - 1.78x  slower
```

After:

```
== Encoding twitter.json (466906 bytes)
ruby 3.4.0preview2 (2024-10-07 master 32c733f57b) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json   224.000 i/100ms
                  oj   230.000 i/100ms
           rapidjson   107.000 i/100ms
Calculating -------------------------------------
                json      2.254k (± 1.6%) i/s  (443.69 μs/i) -     11.424k in   5.069999s
                  oj      2.318k (± 1.4%) i/s  (431.32 μs/i) -     11.730k in   5.060421s
           rapidjson      1.081k (± 1.9%) i/s  (925.05 μs/i) -      5.457k in   5.049738s

Comparison:
                json:     2253.8 i/s
                  oj:     2318.5 i/s - same-ish: difference falls within error
           rapidjson:     1081.0 i/s - 2.08x  slower
```

The escape table is taken directly from Mame's PR.

Co-Authored-By: Yusuke Endoh <mame@ruby-lang.org>

ext/json/ext/generator/generator.c

@@ -117,7 +117,29 @@ static void convert_UTF8_to_JSON(FBuffer *out_buffer, VALUE in_string, bool out_
     RB_GC_GUARD(in_string);
 }
 
-static void convert_ASCII_to_JSON(FBuffer *out_buffer, VALUE str, bool out_script_safe)
+static const bool escape_table[256] = {
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
+    0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* '"' and '/' */
+    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0, /* '\\' */
+    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1
+};
+
+static const bool script_safe_escape_table[256] = {
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
+    0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,1, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* '"' and '/' */
+    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0, /* '\\' */
+    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
+    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1
+};
+
+static void convert_ASCII_to_JSON(FBuffer *out_buffer, VALUE str, const bool escape_table[256])
 {
     const char *hexdig = "0123456789abcdef";
     char scratch[12] = { '\\', 'u', 0, 0, 0, 0, '\\', 'u' };
@@ -129,17 +151,8 @@ static void convert_ASCII_to_JSON(FBuffer *out_buffer, VALUE str, bool out_scrip
 
     for (pos = 0; pos < len;) {
         unsigned char ch = ptr[pos];
-        bool should_escape;
-
-        /* JSON policy */
-        should_escape =
-            (ch < 0x20) ||
-            (ch == '"') ||
-            (ch == '\\') ||
-            (out_script_safe && (ch == '/'));
-
         /* JSON encoding */
-        if (should_escape) {
+        if (escape_table[ch]) {
             if (pos > beg) {
                 fbuffer_append(out_buffer, &ptr[beg], pos - beg);
             }
@@ -717,7 +730,7 @@ static void generate_json_string(FBuffer *buffer, VALUE Vstate, JSON_Generator_S
 
     switch(rb_enc_str_coderange(obj)) {
         case ENC_CODERANGE_7BIT:
-            convert_ASCII_to_JSON(buffer, obj, state->script_safe);
+            convert_ASCII_to_JSON(buffer, obj, state->script_safe ? script_safe_escape_table : escape_table);
             break;
         case ENC_CODERANGE_VALID:
             if (RB_UNLIKELY(state->ascii_only)) {