[flori/json] 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
```

https://github.com/flori/json/commit/a73323dc5e

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