parser.rl: initialize the buffer with 512B on the stack

This very significantly reduce the overhead on smaller benchmarks

```
== Parsing small hash (65 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) [arm64-darwin23]
Warming up --------------------------------------
                json   304.417k i/100ms
                  oj   219.431k i/100ms
           oj strict   254.532k i/100ms
          Oj::Parser   431.309k i/100ms
           rapidjson   281.703k i/100ms
Calculating -------------------------------------
                json      3.046M (± 0.1%) i/s  (328.25 ns/i) -     15.525M in   5.096243s
                  oj      2.225M (± 0.2%) i/s  (449.50 ns/i) -     11.191M in   5.030429s
           oj strict      2.553M (± 0.5%) i/s  (391.75 ns/i) -     12.981M in   5.085538s
          Oj::Parser      4.280M (± 0.8%) i/s  (233.64 ns/i) -     21.565M in   5.038834s
           rapidjson      2.826M (± 0.3%) i/s  (353.83 ns/i) -     14.367M in   5.083480s

Comparison:
                json:  3046420.8 i/s
          Oj::Parser:  4280132.7 i/s - 1.40x  faster
           rapidjson:  2826209.4 i/s - 1.08x  slower
           oj strict:  2552619.7 i/s - 1.19x  slower
                  oj:  2224670.7 i/s - 1.37x  slower
```

ext/json/ext/parser/parser.c

@@ -1919,17 +1919,20 @@ static VALUE cParser_parse(VALUE self)
     VALUE result = Qnil;
     GET_PARSER;
 
+    char stack_buffer[FBUFFER_STACK_SIZE];
+    fbuffer_stack_init(&json->fbuffer, FBUFFER_INITIAL_LENGTH_DEFAULT, stack_buffer, FBUFFER_STACK_SIZE);
 
-#line 1924 "parser.c"
+
+#line 1927 "parser.c"
 	{
 	cs = JSON_start;
 	}
 
-#line 823 "parser.rl"
+#line 826 "parser.rl"
     p = json->source;
     pe = p + json->len;
 
-#line 1933 "parser.c"
+#line 1936 "parser.c"
 	{
 	if ( p == pe )
 		goto _test_eof;
@@ -1973,7 +1976,7 @@ cs = 0;
 	if ( ++p == pe )
 		goto _test_eof10;
 case 10:
-#line 1977 "parser.c"
+#line 1980 "parser.c"
 	switch( (*p) ) {
 		case 13: goto st10;
 		case 32: goto st10;
@@ -2062,7 +2065,7 @@ case 9:
 	_out: {}
 	}
 
-#line 826 "parser.rl"
+#line 829 "parser.rl"
 
     if (cs >= JSON_first_final && p == pe) {
         return result;
@@ -2082,17 +2085,20 @@ static VALUE cParser_m_parse(VALUE klass, VALUE source, VALUE opts)
     JSON_Parser *json = &parser;
     parser_init(json, source, opts);
 
+    char stack_buffer[FBUFFER_STACK_SIZE];
+    fbuffer_stack_init(&json->fbuffer, FBUFFER_INITIAL_LENGTH_DEFAULT, stack_buffer, FBUFFER_STACK_SIZE);
+
 
-#line 2087 "parser.c"
+#line 2093 "parser.c"
 	{
 	cs = JSON_start;
 	}
 
-#line 846 "parser.rl"
+#line 852 "parser.rl"
     p = json->source;
     pe = p + json->len;
 
-#line 2096 "parser.c"
+#line 2102 "parser.c"
 	{
 	if ( p == pe )
 		goto _test_eof;
@@ -2136,7 +2142,7 @@ cs = 0;
 	if ( ++p == pe )
 		goto _test_eof10;
 case 10:
-#line 2140 "parser.c"
+#line 2146 "parser.c"
 	switch( (*p) ) {
 		case 13: goto st10;
 		case 32: goto st10;
@@ -2225,7 +2231,7 @@ case 9:
 	_out: {}
 	}
 
-#line 849 "parser.rl"
+#line 855 "parser.rl"
 
     if (cs >= JSON_first_final && p == pe) {
         return result;

ext/json/ext/parser/parser.rl

@@ -819,6 +819,9 @@ static VALUE cParser_parse(VALUE self)
     VALUE result = Qnil;
     GET_PARSER;
 
+    char stack_buffer[FBUFFER_STACK_SIZE];
+    fbuffer_stack_init(&json->fbuffer, FBUFFER_INITIAL_LENGTH_DEFAULT, stack_buffer, FBUFFER_STACK_SIZE);
+
     %% write init;
     p = json->source;
     pe = p + json->len;
@@ -842,6 +845,9 @@ static VALUE cParser_m_parse(VALUE klass, VALUE source, VALUE opts)
     JSON_Parser *json = &parser;
     parser_init(json, source, opts);
 
+    char stack_buffer[FBUFFER_STACK_SIZE];
+    fbuffer_stack_init(&json->fbuffer, FBUFFER_INITIAL_LENGTH_DEFAULT, stack_buffer, FBUFFER_STACK_SIZE);
+
     %% write init;
     p = json->source;
     pe = p + json->len;