Improve JSON_parse_string() performance

`rb_str_resize()` might reallocate the heap area in String object and it might cause to downgrade the performance.

When use `rb_str_new()` instead of `rb_str_buf_new()`, it does not allocate extra heap area internally. After then, it will not need to resize the object.

This patch will be 36.6 % faster for parsing string value.

## Before
```
$ ruby bench.rb
Warming up --------------------------------------
                json    14.000  i/100ms
Calculating -------------------------------------
                json    144.882  (± 1.4%) i/s -    728.000  in   5.025682s
** Memory usage
1105348
```

## After
```
$ ruby bench.rb
Warming up --------------------------------------
                json    19.000  i/100ms
Calculating -------------------------------------
                json    197.928  (± 4.5%) i/s -    988.000  in   5.005914s
** Memory usage
1138428
```

## Test code
```
require 'json'
require 'objspace'
require 'securerandom'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id": i,
    "uuid": SecureRandom.uuid,
    "created_at": Time.now
  }
end

json = obj.to_json

Benchmark.ips do |x|
  x.report "json" do |iter|
    count = 0
    while count < iter
      JSON.parse(json)
      count += 1
    end
  end
end

puts "** Memory usage"
p ObjectSpace.memsize_of_all String
```

ext/json/ext/parser/parser.c

@@ -1514,7 +1514,7 @@ static char *JSON_parse_string(JSON_Parser *json, char *p, char *pe, VALUE *resu
     int cs = EVIL;
     VALUE match_string;
 
-    *result = rb_str_buf_new(0);
+    *result = rb_str_new(NULL, 0);
 
 #line 1520 "parser.c"
 	{
@@ -1658,8 +1658,6 @@ case 7:
 
     if (json->symbolize_names && json->parsing_name) {
       *result = rb_str_intern(*result);
-    } else {
-      rb_str_resize(*result, RSTRING_LEN(*result));
     }
     if (cs >= JSON_string_first_final) {
         return p + 1;
@@ -1830,15 +1828,15 @@ static VALUE cParser_initialize(int argc, VALUE *argv, VALUE self)
 }
 
 
-#line 1834 "parser.c"
+#line 1832 "parser.c"
 enum {JSON_start = 1};
 enum {JSON_first_final = 10};
 enum {JSON_error = 0};
 
 enum {JSON_en_main = 1};
 
 
-#line 742 "parser.rl"
+#line 740 "parser.rl"
 
 
 /*
@@ -1855,16 +1853,16 @@ static VALUE cParser_parse(VALUE self)
   GET_PARSER;
 
 
-#line 1859 "parser.c"
+#line 1857 "parser.c"
 	{
 	cs = JSON_start;
 	}
 
-#line 758 "parser.rl"
+#line 756 "parser.rl"
   p = json->source;
   pe = p + json->len;
 
-#line 1868 "parser.c"
+#line 1866 "parser.c"
 	{
 	if ( p == pe )
 		goto _test_eof;
@@ -1898,7 +1896,7 @@ case 1:
 cs = 0;
 	goto _out;
 tr2:
-#line 734 "parser.rl"
+#line 732 "parser.rl"
 	{
         char *np = JSON_parse_value(json, p, pe, &result, 0);
         if (np == NULL) { p--; {p++; cs = 10; goto _out;} } else {p = (( np))-1;}
@@ -1908,7 +1906,7 @@ cs = 0;
 	if ( ++p == pe )
 		goto _test_eof10;
 case 10:
-#line 1912 "parser.c"
+#line 1910 "parser.c"
 	switch( (*p) ) {
 		case 13: goto st10;
 		case 32: goto st10;
@@ -1997,7 +1995,7 @@ case 9:
 	_out: {}
 	}
 
-#line 761 "parser.rl"
+#line 759 "parser.rl"
 
   if (cs >= JSON_first_final && p == pe) {
     return result;

ext/json/ext/parser/parser.rl

@@ -535,7 +535,7 @@ static char *JSON_parse_string(JSON_Parser *json, char *p, char *pe, VALUE *resu
     int cs = EVIL;
     VALUE match_string;
 
-    *result = rb_str_buf_new(0);
+    *result = rb_str_new(NULL, 0);
     %% write init;
     json->memo = p;
     %% write exec;
@@ -553,8 +553,6 @@ static char *JSON_parse_string(JSON_Parser *json, char *p, char *pe, VALUE *resu
 
     if (json->symbolize_names && json->parsing_name) {
       *result = rb_str_intern(*result);
-    } else {
-      rb_str_resize(*result, RSTRING_LEN(*result));
     }
     if (cs >= JSON_string_first_final) {
         return p + 1;