Convert String encoding using `rb_str_encode()`

`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
```

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