Ignoring `CHAR_BITS` > 8 platform, as far as `ch` indexes
`escape_table` that is hard-coded as 256 elements.

```
../../../../src/ext/json/generator/generator.c(121): warning C4333: '>>': right shift by too large amount, data loss
../../../../src/ext/json/generator/generator.c(122): warning C4333: '>>': right shift by too large amount, data loss
../../../../src/ext/json/generator/generator.c(243): warning C4333: '>>': right shift by too large amount, data loss
../../../../src/ext/json/generator/generator.c(244): warning C4333: '>>': right shift by too large amount, data loss
../../../../src/ext/json/generator/generator.c(291): warning C4333: '>>': right shift by too large amount, data loss
../../../../src/ext/json/generator/generator.c(292): warning C4333: '>>': right shift by too large amount, data loss
```

https://github.com/ruby/json/commit/fb82373612