## Before
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    25.000  i/100ms
Calculating -------------------------------------
                json    250.478  (± 4.8%) i/s -      1.250k in   5.002238s
```

## After
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    32.000  i/100ms
Calculating -------------------------------------
                json    360.652  (± 3.6%) i/s -      1.824k in   5.064511s
```

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

obj = []

1000.times do |i|
  obj << {
    :string => "x" * 100,
    :utf8 => "あ" * 100
  }
end

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

## Before
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json   129.000  i/100ms
Calculating -------------------------------------
                json      1.300k (± 2.3%) i/s -      6.579k in   5.064656s
```

## After
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json   189.000  i/100ms
Calculating -------------------------------------
                json      1.964k (± 3.3%) i/s -      9.828k in   5.011237s
```

## Code
```
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
```

`rb_funcall` might be slightly heavy to call the Ruby method.
This patch will convert String encoding using `rb_str_encode()` instead of `rb_funcall()`.

## Before
```
$ 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
```

## After
```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json   129.000  i/100ms
Calculating -------------------------------------
                json      1.300k (± 2.3%) i/s -      6.579k in   5.064656s
```

## Code
```
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
```

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
```

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
```