Convert Hash object using rb_hash_foreach()

To convert Hash convert, this part was using following pseudo code

```
obj.keys.each do |key|
  value = obj[key]
  ...
end
```

and `rb_funcall()` was called for `obj.keys`.
It might be slightly heavy to call the Ruby method.
This patch will iterate to convert Hash object about key/value using `rb_hash_foreach()` Ruby API instead of `rb_funcall()`.

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    55.000  i/100ms
Calculating -------------------------------------
                json    558.501  (± 1.1%) i/s -      2.805k in   5.022986s
```

```
$ 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
```

```
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

@@ -715,43 +715,76 @@ static VALUE cState_aset(VALUE self, VALUE name, VALUE value)
     return Qnil;
 }
 
-static void generate_json_object(FBuffer *buffer, VALUE Vstate, JSON_Generator_State *state, VALUE obj)
+struct hash_foreach_arg {
+    FBuffer *buffer;
+    JSON_Generator_State *state;
+    VALUE Vstate;
+    int iter;
+};
+
+static int
+json_object_i(VALUE key, VALUE val, VALUE _arg)
 {
+    struct hash_foreach_arg *arg = (struct hash_foreach_arg *)_arg;
+    FBuffer *buffer = arg->buffer;
+    JSON_Generator_State *state = arg->state;
+    VALUE Vstate = arg->Vstate;
+
     char *object_nl = state->object_nl;
     long object_nl_len = state->object_nl_len;
     char *indent = state->indent;
     long indent_len = state->indent_len;
-    long max_nesting = state->max_nesting;
     char *delim = FBUFFER_PTR(state->object_delim);
     long delim_len = FBUFFER_LEN(state->object_delim);
     char *delim2 = FBUFFER_PTR(state->object_delim2);
     long delim2_len = FBUFFER_LEN(state->object_delim2);
+    long depth = state->depth;
+    int j;
+    VALUE key_to_s;
+
+    if (arg->iter > 0) fbuffer_append(buffer, delim, delim_len);
+    if (object_nl) {
+        fbuffer_append(buffer, object_nl, object_nl_len);
+    }
+    if (indent) {
+        for (j = 0; j < depth; j++) {
+            fbuffer_append(buffer, indent, indent_len);
+        }
+    }
+
+    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);
+    generate_json(buffer, Vstate, state, val);
+
+    arg->iter++;
+    return ST_CONTINUE;
+}
+
+static void generate_json_object(FBuffer *buffer, VALUE Vstate, JSON_Generator_State *state, VALUE obj)
+{
+    char *object_nl = state->object_nl;
+    long object_nl_len = state->object_nl_len;
+    char *indent = state->indent;
+    long indent_len = state->indent_len;
+    long max_nesting = state->max_nesting;
     long depth = ++state->depth;
-    int i, j;
-    VALUE key, key_to_s, keys;
+    int j;
+    struct hash_foreach_arg arg;
+
     if (max_nesting != 0 && depth > max_nesting) {
         fbuffer_free(buffer);
         rb_raise(eNestingError, "nesting of %ld is too deep", --state->depth);
     }
     fbuffer_append_char(buffer, '{');
-    keys = rb_funcall(obj, i_keys, 0);
-    for(i = 0; i < RARRAY_LEN(keys); i++) {
-        if (i > 0) fbuffer_append(buffer, delim, delim_len);
-        if (object_nl) {
-            fbuffer_append(buffer, object_nl, object_nl_len);
-        }
-        if (indent) {
-            for (j = 0; j < depth; j++) {
-                fbuffer_append(buffer, indent, indent_len);
-            }
-        }
-        key = rb_ary_entry(keys, i);
-        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);
-        generate_json(buffer, Vstate, state, rb_hash_aref(obj, key));
-    }
+
+    arg.buffer = buffer;
+    arg.state = state;
+    arg.Vstate = Vstate;
+    arg.iter = 0;
+    rb_hash_foreach(obj, json_object_i, (VALUE)&arg);
+
     depth = --state->depth;
     if (object_nl) {
         fbuffer_append(buffer, object_nl, object_nl_len);