Nested mixes are duplicated

```js
({
block: 'b1',
mix: 'b2'
})
```

```js
block('b2').addMix()('b3')
```

http://bem.github.io/bem-xjst/?version=8.9.1&engine=BEMHTML&template=block(%27b2%27).addMix()(%27b3%27)&bemjson=(%7B%0A%20%20%20%20block%3A%20%27b1%27%2C%0A%20%20%20%20mix%3A%20%27b2%27%0A%7D)

Open the linked bem-xjst reproduction and run the shown BEMJSON with the BEMHTML template. Trace how the nested mix is represented and rendered; done means the b2 mix is emitted only once in the resulting output.

Written by the indexing model from the issue text.