Add shortcut converting to String

In where to convert Hash key to String for json, this patch will add shortcut for String/Symbol in Hash key.

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    65.000  i/100ms
Calculating -------------------------------------
                json    659.576  (± 1.5%) i/s -      3.315k in   5.027127s
```

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    78.000  i/100ms
Calculating -------------------------------------
                json    789.781  (± 2.7%) i/s -      3.978k in   5.041043s
```

```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id" => i,
    :age => 42,
  }
end

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

ext/json/ext/generator/generator.c

@@ -740,7 +740,7 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
     long delim2_len = FBUFFER_LEN(state->object_delim2);
     long depth = state->depth;
     int j;
-    VALUE key_to_s;
+    VALUE klass, key_to_s;
 
     if (arg->iter > 0) fbuffer_append(buffer, delim, delim_len);
     if (object_nl) {
@@ -752,7 +752,14 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
         }
     }
 
-    key_to_s = rb_funcall(key, i_to_s, 0);
+    klass = CLASS_OF(key);
+    if (klass == rb_cString) {
+        key_to_s = key;
+    } else if (klass == rb_cSymbol) {
+        key_to_s = rb_id2str(SYM2ID(key));
+    } else {
+        key_to_s = rb_funcall(key, i_to_s, 0);
+    }
     Check_Type(key_to_s, T_STRING);
     generate_json(buffer, Vstate, state, key_to_s);
     fbuffer_append(buffer, delim2, delim2_len);