[ruby/json] Ensure `Generator::State` is kept on the stack

Fix: https://github.com/ruby/json/issues/929

When calling `cState_partial_generate` from `mHash_to_json` or other
`to_json` funcs, `VState` becomes unreachable very quickly, hence the
compiler may optimize it out of the stack, and make it invisible to
the GC stack scanning. This is particularly liekly given how aggressively
we inline.

Repro:

```ruby
require 'json'

test_data = {
  "flag" => true,
  "data" => 10000.times.map { [1.0] },
  :flag => false,
}

10.times do
  test_data.to_json
end
```

But in practice the cause was just that the issued warning calls
Hash#inspect on a big hash, which triggers GC.

So it can be triggered even more reliably with:

```ruby
require 'json'

module JSON
  module Common
    def self.on_mixed_keys_hash(...)
      GC.start
    end
  end
end

test_data = {
  "flag" => true,
  "data" => 10000.times.map { [1.0] },
  :flag => false,
}

test_data.to_json
```

https://github.com/ruby/json/commit/79b6e168ba

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